Ocean in Crisis

The Challenger Society for Marine Science was founded in 1903 by Herbert Fowler and Richard Norris Wolfenden, like-minded marine biologists determined to honour Challenger’s legacy by establishing oceanography to its rightful place within the scientific community. 

Today, through the society’s periodical, Ocean Challenge, conferences, working groups and support for young scientists, it remains at the forefront of marine science.

The impacts of climate change on the oceans and marine life are of growing concern to Society and its members. 

 There is an increasing awareness, explored here, of the changes that are occurring to the seas and ocean as a result of human activities. Unsustainable fishing practices, the extraction of minerals and energy and the presence of contaminants are all contributing to the changes. 

The increase in greenhouse gas concentrations in the atmosphere is having a significant impact on the ocean because it is absorbing a significant proportion of both the additional CO2 and the resulting extra heat. There is also a potential for changes in the currents that transport heat, salt and planktonic organisms, within and between ocean basins. All this presents a worrying picture for the future. 

As many are now recognising, Challenger’s extraordinary 150-year-old archive has powerful new relevance today in our understanding of the impacts of climate change on the ocean and its marine life.

Ocean Science

The Challenger Society (1903)

‘Two little Babes on the Beach’ 

According to Margaret Deacon’s history of the society, one of its founders, Herbert Fowler, once said:

When we read papers to one of the established Societies, we had the feeling that no one present seemed to care about marine work. We felt like two little Babes on the Beach picking up seaweed and dead shrimps.

Another founder member, Challenger scientist, Sir John Murray, had edited the official 30,000-word Expedition Report, completing the work of the late Professor Wyville Thomson. Murray believed that the expedition’s findings provided

the greatest advance in the knowledge of our planet since the celebrated discoveries of the fifteenth and sixteenth centuries.

Read more about Origins of the Challenger Society

Science of the Sea Handbook, 1912


From the very start, the Society aimed ‘to advance the study and application of the marine sciences through research and education.’ The provision of a forum for marine science was important, but this was not intended to be the Society’s only function. In an early initiative, they published Science of the Sea (1912), a handbook designed to be useful to people not necessarily trained in science: travellers, sailors, yachtsmen and the like, who had opportunities for making observations at sea.


Science of the Seas, Preface

The handbook opens with a compelling claim for the global significance of the world’s oceans:

Not only the importance, but even the very shape of the water is apt to be overlooked, a neglect fostered by atlases which, as a rule, are concerned merely with the land. Yet the surface of the Earth shows about two and a half times as much water as land. The ordinary atlas fails to show that the land is massed towards the North Pole, the water towards the South [see diagram below].

Still less do ordinary maps display the connection between the great oceans: the [waters of the] South pass into a blank Unknown. It is not easy to get a proper grasp of this connection…but several of the star projections show fairly well that the three great oceans – Pacific, Atlantic, and Indian – can be regarded as bays of that single Southern Ocean which forms a wide belt round the Antarctic Continent… 

‘The three great oceans – Pacific, Atlantic, and Indian – are ‘bays of that single Southern Ocean.’ Source: Science of the Seas.

Still less do we know of the details of the inner working of those stupendous phenomena which affect five-sevenths of the Earth’s surface. At the other end of the scale, so far as mere size is concerned, we do not at present really understand the life of a single rock-pool, the delicate adjustment which exists between the myriads of organisms, its inhabitants.

Equipment advertised in Science of the Sea.

Enlisting the help of volunteers was an early ambition. Science of the Sea consisted of chapters on different aspects of marine, explaining the marine sciences and the equipment and methods employed in their study.

Equity at Sea

But Margaret Deacon’s account also acknowledges that the ‘Admission of women took rather longer. Their exclusion had been deliberate from the start, in spite of the fact that (with the exception of ocean-going cruises) they were active in all aspects of marine science, and especially in marine biology. Most other scientific societies had women members. The Challenger Society, however, maintained its policy of exclusion until the Royal Society (where it customarily met) itself lifted its ban after the Second World War.’

This ‘hurtful policy’ stemmed principally from the desire for more informality at meetings than would have been possible under the rules of Edwardian etiquette if both sexes had been present. It was to some extent mitigated by the introduction of joint meetings with other organisations in the 1920s.

But today, as Ocean Challenge argues in Equity at Sea,

we can celebrate a strong representation of women in sea-going science in the United Kingdom, providing positive role models for early-career female marine scientists. However, women continue to face challenges to their progression in their marine science careers, especially those who are also members of other under-represented groups.

The history of sea-going women in research begins in 1766, when Jeanne Baret (1740–1807), dressed as a teenage boy, joined expeditions as assistant to the naturalist Philibert Commerçon on board the ships La Boudeuse and L’Étoile. The French botanist became the first known sea-going woman scientist, was the first to reach the Antarctic.


For more on Equity at Sea read:

The Challenger Society’s Ocean Challenge Periodical No. 24



Mapping the deep: from Sir John Murray to Marie Tharp

It has been more than 100 years since the publication of Sir John Murray’s ‘bathymetrical chart’ of the Atlantic Ocean. Murray compiled his map from lead-line surveys during expeditions such as those of Challenger (on which he sailed) and the ‘Michael Sars’ North Atlantic Deep-Sea Expedition (1910) , as below. But, as we shall see, no truly detailed map of the ocean floor’s topography was available until 1977, when Marie Tharp published the first underwater map of the world.

The coloured contour map revealed for the first time the nature of Earth’s surface beneath the oceans and the outline of the continental margins, the Mid-Atlantic Ridge and the intervening abyssal plains, labelled ‘Deeps.’ Some of the ocean Deeps are named after the captain and scientists aboard Challenger, including Captain Nares, Herbert Moseley and John Buchanan.

Source: Science, seamounts and society, The Geological Society.

Murray’s seafloor profiles showed, however, that apart from the prominence of a few scattered islands such as the Azores, the seafloor of the oceans was smooth and featureless, a view that persisted for about the next four decades. Since then, surveys have revealed many thousands of sub-surface sea mounts, many volcanic in origin, across the ocean basins.

Marie Tharp’s groundbreaking maps brought the seafloor to the world

Completed in 1977, the map represents the culmination of the unlikely, and underappreciated, career of Marie Tharp. Her three decades of work as a geologist and cartographer at Columbia University gave scientists and the public alike their first glimpse of what the seafloor actually looks like.

Source: Sciencenews.

Barred as women then were from ocean expeditions, Tharp poured all of her energy into mapping the seafloor starting with the North Atlantic. To make a map, she translated the echo soundings gathered by ships crossing the ocean into depths and then created two-dimensional vertical slices of the terrain beneath the ships’ tracks.

These ocean-floor profiles showed a broad ridge running down the middle of the Atlantic. Though the feature had been roughly mapped by Challenger in the 19th century, Tharp noticed a notch near the top of the ridge in each of the profiles. She believed the notches represented a continuous, deep valley running down the centre of the mid-ocean ridge. If she was right, the valley might be a rift where molten material came up from below, forming new crust and pushing the ocean floor apart — evidence that could support continental drift.

In the middle of the 20th century, when many American scientists were in revolt against continental drift — the controversial idea that the continents are not fixed in place — Tharp’s groundbreaking maps helped tilt the scientific view toward acceptance and clear a path for the emerging theory of plate tectonics.

Marie Tharp

Tharp was the right person in the right place at the right time to make the first detailed maps of the seafloor. Specifically, she was the right woman. Her gender meant certain professional avenues were essentially off-limits. But she was able to take advantage of doors cracked open by historical circumstances, becoming uniquely qualified to make significant contributions to both science and cartography. Without her, the maps may never have come to be.

Science, seamounts and society: https://www.geolsoc.org.uk/Geoscientist/Archive/August-2019/Feature-1

Marie Tharp’s groundbreaking maps brought the seafloor to the world: https://www.sciencenews.org/article/marie-tharp-maps-plate-tectonics-seafloor-cartography

Our Ocean in Crisis

Deep sea mining imperils 'our greatest ally', the Ocean

Mining companies are negotiating controversial permits from the UN’s International Seabed Authority (ISA) to strip mine metallic nodules on the deep Ocean floor for the copper, cobalt and nickel they contain. Marine scientists, governments and environmental bodies believe that deep seabed mining will worsen the climate crisis, by destroying the natural carbon storage capacity of the deep Ocean, and disrupt marine cycles that add to these carbon stores.

A new report, Undisturbed: the deep Ocean’s role in safeguarding us from the climate crisis, calls on governments and civil society organizations, both at COP27 and at the ISA negotiations in Jamaica (November 2022), to stop the miners in their tracks.


In her book, The brilliant abyss marine scientist Helen Scales argues that ‘the ISA is tragically failing in its responsibilities to safeguard life in the abyss, not to mention threatening the rest of the planet.’ 

Yet the UK government, sponsor of commercial deep sea mining, has repeatedly resisted calls to halt its development. 

So, why the extractivist drive to strip mine the Ocean floor? And what is at stake?

Our common heritage

Across the plains of the deep Ocean, at depths of 4000-5000 meters, metal-bearing nodules, rich in cobalt, manganese, copper and nickel lie close together much like potatoes on a field.

The knuckle-shaped nodules form slowly over millions of years by the precipitation of metal ions in seawater around objects on the sea floor, like a shark’s tooth or clam shell – just like a pearl forms around a grain of sand. A nodule on display at the Natural History Museum, London developed around the tooth of a long-dead Megalodon, a giant shark extinct for three million years.

Ironically, the expanding green economy is boosting demand for these metals, for  batteries, electric cars, parts for solar panels, wind turbines, as well as mobile phones and other new technologies: over six million electric vehicles were sold last year, and sales may triple by 2025

‘What’s yours is mine’

Marine biologist Helen Scales argues that,

‘The proposed mining ventures bear the hallmarks of extractivism, a centuries-old economic model commonly associated with colonialism and latterly with transnational corporations that extract natural raw materials for export. The goal is to mine a resource on a one-shot basis, then move on elsewhere and repeat.’

The Metals Company, one of the main companies pushing deep sea mining, has been testing mining equipment in the depths of the Pacific Ocean since mid-September. According to its CEO, Gerrard Barron, ‘The nodules are literally sitting there like golf balls on a driving range…[during mining] micro-organisms on the seabed are not destroyed. Instead, they are shaken up and carry on.’ His company has negotiated numerous exploration pilot licences in extractive partnerships with the Pacific island states of Nauru, Tonga and Kiribati.

The UK government sponsors two deep sea mineral exploration contracts in the Pacific, via UK Seabed Resources, a wholly-owned subsidiary of the weapons company Lockheed Martin. UKSR claims,  ‘Seabed harvesting is an ecologically sound method for meeting the growing global demand for precious metals.’

The Ocean: our buffer against climate change

However, a joint study by the Scripps Institution of Oceanography, and the International Programme on the State of the Ocean highlights the vital role the deep Ocean plays in the functioning of our planet, including ‘buffering’ the impacts climate change. Their report, Undisturbed says:

‘The bonds between The Ocean and climate change run deep. The deep Ocean helps to regulate Earth’s climate by absorbing and storing over 90% of the excess heat and approximately 38% of the carbon dioxide generated by humanity.’ A new extractive deep-sea industry would lead to biodiversity loss and disruption of ecosystem services on an enormous scale. ‘Widespread destruction of the seafloor would bring long-term impacts on carbon cycling and storage in the deep.’

Strip mining the seabed will add significantly to the multiple threats already facing the deep Ocean, including rising temperatures and acidity, species extinction, glacier melt, micro-plastic pollution and over fishing.

How deep sea mining works

Source: EU mining impact studies

Mining pilot videos depict massive 16-meter wide bulldozers lowered from a mother ship to 4000-5000 meters. They scoop up the top 15 cm of sea floor sediments, nodules and all marine life in their path. The nodules are separated from the ‘slurry,’ which is blasted out the back of the machine in a continuous plume. The nodules are sucked up a riser pipe to the ship. Waste material is dumped back in the sea. Flora and fauna accustomed to a dark, near-silent environment face a 24/7 regime of machine noise, with floodlights for remote monitoring.

Over a two-week period, an estimated 100,000 tonnes of material could be removed. A 30-year operating licence would strip 10,000 square kilometres of the seabed.

Deep sea mining: environmental impacts

Deep sea mining impact studies sponsored by the EU concluded that ‘metal nodule ecosystems support a unique and highly diverse fauna of static and mobile species.’ Impacts of ‘even small-scale experimental seafloor disturbances on nodule habitats last for many decades and affected numerous ecosystems.’

For Prof. Philip Weaver of the National Oceanography Centre, ‘The highest priority is to determine the response of organisms to the impacts of plumes, including their particle load and toxicity.’ Toxic-laden particles in the slurry blasted out the back of the bulldozers can smother, choke, harm or kill animal and plant life and pollute pristine marine environments over tens or hundreds of kilometres around the strip mine. Anemones, sponges, corals, nematode worms and microscopic tardigrades (aka ‘water bears’) attach themselves to the nodules, and are more abundant than in areas without nodule coverage.

Issue Environmental and climate change impacts
Carbon absorption and storage Widescale disturbance of carbon sinks (marine sediments storing carbon emissions).  Significant effects on marine species’ carbon cycling and storage processes.
Loss of species, removal of rocks and other matter hosting organisms

Removal and destruction of seafloor habitats along with their unique fauna. Biodiversity loss, ecosystem disruption, migration barriers for many species. Loss of many as yet unknown species. 

Deposits from widely dispersed vehicle ‘slurry’ takes tens to hundreds of years to settle and consolidate.

Impacts of exhaust plumes Exhaust plumes spread many kilometres beyond mined area, impacting vulnerable species on the seabed or in the water.
Waste matter Waste material discarded overboard impacts on free-floating organisms.
Fauna recovery time Many decades to hundreds of years for the recovery of surviving deep sea fauna, including corals and sponges.

Time to ‘Stop’ mining in its tracks

The United Nations is responsible for the regulation and stewardship of international waters through UN Convention on the Law of the Sea (1982) ‘on behalf of all humankind.’  Article 136 defines the deep ocean and its resources as ‘the common heritage of mankind.’ The UN delegated the development of rules and regulations for the effective protection of the marine environment to the International Seabed Authority (ISA).  https://www.isa.org.jm/

Controversially, the ISA is currently (November 2022) drafting regulations covering

‘exploration and exploitation’ of mineral resources on the Ocean floor. It handed this task to an obscure, 30-member Legal and Technical Commission, often meeting behind closed doors. The ISA’s incumbent General Secretary in 2022, Michael Lodge, has shown ample signs of his pro-mining stance.

Under a two-year rule triggered in July 2021 by the Pacific island nation of Nauru, exploitation of the seabed could commence by July 2023 even if environmental or economic regulations have not been agreed. Greenpeace estimates that ‘contracts to explore for deep sea mining potential covering over a million square kilometres of the international seabed have been given out by the ISA.’

The Metals Company’s sponsorship deals with Pacific island nations Nauru, Tonga and Kiribati has secured exploration rights to 150,000 square kilometers of seabed. The company’s $2.9 billion value vastly exceeds the combined GDPs of its three ‘partners.’ In Nauru’s case, much of the small Pacific island has been devastated by phosphate mining. Its riches squandered, the island contracted to host Australia’s ‘offshore’ refugee detention centre. Seabed mining is the latest idea for digging the country out of financial trouble. 

In Jamaica, the Deep Sea Conservation Coalition of NGOs, youth groups, political leaders, MPs, technology and car companies, and scientists has urged the Authority to ‘stop to the destructive industry before it starts.’ Some ISA member states, including Germany, Spain and New Zealand, are calling for a ‘pause and reflection’ over the whole deep sea mining process.


‘The whole global mindset on recycling of metals has to change,’ says the UN’s Environment Programme (UNEP). Mobile phones contain more than 40 metal elements, including copper, tin and cobalt, but recycling rates are minimal. UNEP is calling for ‘Policy and legislation to mandate product recycling standards and incentives.’  In the UK, with the exception of steel, metals recycling rates are low across the board. The UK’s critical minerals strategy offers no new legislation to improve recycling and recovery. Yet it sponsors deep sea mining.

Greenpeace says:

Governments have been far too passive in holding the ISA to account. While they are negotiating at COP 27 to combat the climate crisis, an obscure international regulator is undermining their efforts by advancing deep sea mining. During this round of negotiations delegates must reaffirm the commitments they have made to protecting the ocean and truly preserve it as the common heritage of humankind.

How to find out more: https://www.greenpeace.org/international/act/stop-deep-sea-mining/



Helen Scales, The brilliant abyss, (2021)


Chapter 10 exposes the ISA’s controversial drive to facilitate deep sea mining. 

Greenpeace: Deep Trouble – The murky world of the deep sea mining industry: https://tinyurl.com/2zhet822

Undisturbed: the deep Ocean’s vital role in safeguarding us from crises:  


State of negotiations: https://tinyurl.com/2bxnj33a 

Philip Weaver and others, Assessing plume impacts caused by nodule mining vehicles


Flora and fauna International: https://tinyurl.com/4akfhxtu

EU Joint Programme Initiative, Ecological Aspects of Deep-Sea Mining:


The Metals Company:




Beginnings: Challenger Sounding Station 272

It’s a remarkable story, really. On 8 September 1875, Challenger reached a point in the middle of the Pacific Ocean, Sounding Station number 272, where the scientists and crew began another day’s work. Here, they lowered their equipment into the sea to record the ocean’s depth (2,600 fathoms, or 15,600 feet), trawl the sea floor for marine specimens, and test the water temperature and salinity. Their findings were unique enough at the time. But today, 150 years later, their simple but pioneering, hand-made measurements have become benchmarks to study changes to marine life, ocean acidity and ocean temperatures, driven by climate change and man-made carbon emissions since the industrial revolution.

To gather samples that day in 1875, the crew lowered a cable nearly three miles long to the sea floor. Weights, sampling bottles, thermometers and a trawling bag were attached to the line, with coloured flags at intervals to count out the length of line the crew paid out.

Sounding Station 272, mid-Pacific Ocean, one of Challenger’s 354 sounding points.

Source: Nature magazine, 2020.

If there was a sea breeze, the steam engine held the ship steady, head to wind:

A workable and more reliable dredging routine was soon established. Steam power was an essential part of the operation. ‘The first thing to be done,’ writes engineer Spry, ‘is to shorten and furl all sail and bring the ship head to wind, regulating her engine speed in such a manner as to avoid forcing her through water.’ With the ship held steady in position at the sounding station, the screw turning slowly, work could then begin.


Sub-Lieutenant Swire led the sounding operations designed to measure the depth and other conditions of the oceans. The sailors assembled the inch-thick depth-sounding lines, stacking them on deck in reels 3000-fathoms long (about three and a half miles).


Source: A Challenger’s Song, page 123

Dredging and Sounding arrangements on board the Challenger

Scientists at work aboard ship. Challenger Report

Today: Climate change, marine life in the acidic ocean

Scientists at the Natural History Museum have now re-examined a clutch of Challenger’s specimens collected 150 years ago at Station 272, located in the mid-Pacific Ocean. The specimens they reviewed are foraminifera, minute, millimeter-sized single cell organisms with a thin shell usually made from calcium carbonate, such as these.

Source: British geological Survey

Ocean acidification is caused by the rapidly increasing concentration of carbon (CO2) in the atmosphere due to human activity. As CO2 levels continue to rise, more of the gas dissolves into the world’s oceans. The waters become more acidic, and some of the smallest yet most important organisms in our ocean’s food chain are beginning to struggle.

During the expedition, Challenger scientists routinely stored and labelled their specimens in sample jars such as the one shown here. The sample jars are kept at the Natural History Museum, London.

Comparing the Challenger’s findings of specimens that were alive at the time they were retrieved (8 September in 1875) with modern samples taken from the same part of the Pacific Ocean by the Tara Expeditions  (2009–2016), a team of the museum’s scientists found that:

  • single-celled organisms (known as planktonic foraminifera) are now failing to build shells of the same thickness, due to increasing ocean acidity.
  • all modern specimens had up to 76% thinner shells than their historic counterparts, which corresponds to a period of profound change in our oceans.

 Challenger specimen: Neogloboquadrina acostaensis.

According to Dr Lyndsey Fox, who led the scientific team, ‘There have been dramatic reductions in shell thickness in some species of foraminifera, though less so in others.’ This Challenger specimen is Neogloboquadrina acostaensis. In this Nano-CT scan, warm colours indicate areas of relatively thicker shell.


Tara Expeditions vessel

Re-examining the Challenger specimens and the scientists’ own Sounding notebooks provide the first direct evidence of how ocean acidification is impacting carbonate (shell) producing marine life over a long time period in modern oceans.

Challenger Sounding books, Natural History Museum: #ChallengerRevisited

A Challenger sample jar. It contains specimens from Sample Station 302, located on the ‘East Coast of S. America,’ dated 28 December 1875. Natural History Museum


Nature, January 2020, Quantifying the Effects of Climate Change on Plankton

Today: Rising ocean temperatures

The Challenger expedition provides direct evidence of rising ocean temperatures.

The longest interval over which records of ocean temperatures can be compared on a global scale is the 135 years between the voyage of HMS Challenger (1872-1876) and the modern data set of the Argo Programme (2004–2010), whose floating devices continuously measure the temperature and salinity of the ocean. The study, reported in Nature, underlines the scientific significance of the Challenger expedition and the modern Argo Programme and indicates that globally the oceans have been warming at least since the late-nineteenth or early-twentieth century. https://www.nature.com/articles/nclimate1461

During the Challenger’s four-year expedition it stopped 354 times to take soundings of ocean temperature and salinity. For example, on September 8th 1875 at Sounding Station number 272 (see chart), the ocean’s surface temperature was 79 degrees Fahrenheit  (26.1 degrees Celsius), and on the sea floor the temperature measured 35.1 degrees Fahrenheit (1.7 degrees C), at a depth of 2,600 fathoms (15,500 feet).

Compared with Challenger’s sparse but pioneering measurements, across the globe today some 4,000 Argo floats continuously collect and transmit data on the temperature and salinity of the ocean. Once deployed, an Argo floats, sinks to 1000 meters, drifts with the ocean currents, sinks a further 1000 meters, and then resurfaces to transmit its data to a satellite. The batteries last four years.

 Argo floats: Ocean Challenge, 2019, vol. 23 no.2.

The study, the first global-scale comparison of Challenger and modern data, shows average warming at the surface waters of the ocean of some 0.6 degrees Centigrade. Warming in the Atlantic Ocean was found to be stronger than in the Pacific Ocean.

 Challenger Soundings, August-September 1875: Challenger Report.


Report of Challenger expedition: https://www.19thcenturyscience.org/HMSC/HMSC-Reports/1885-Narrative/htm/doc1014.html

135 years of global ocean warming between the Challenger expedition and the Argo Programme: https://www.nature.com/articles/nclimate1461

From HMS Challenger to Argo and beyond, Judith Wolf and Colin Pelton, Ocean Challenge, 2019: https://www.challenger-society.org.uk/oceanchallenge/2019_23_2.pdf

Planet Ocean: climate change and Antarctic waters

‘How inappropriate to call this planet Earth when it is quite clearly Ocean.’ This quote, beloved of oceanographers and others who care about the sea, comes from science fiction writer Arthur C. Clarke. For Mike Meredith of the British Antarctic Survey, the notion ‘encapsulates perfectly the pre-eminence of the ocean in everything to do with our planet – sustaining life, controlling our climate, feeding our populations.’ 

He describes the currents of the Southern Ocean as driving the global circulation of ocean waters. They flow round Antarctica, connecting with the Atlantic, Pacific and Indian ocean basins. ‘They move huge quantities of heat and fresh water, along with carbon and other climatically and ecologically important substances between them.’

As Meredith shows https://www.challenger-society.org.uk/oceanchallenge/2019_23_2.pdf, the idea of a single inter-connected ocean with many features becomes clear visually when the world is viewed through the ‘Spilhaus’ projection:



A cross-section of the Southern Ocean shows how the reprocessing of deep water to form intermediate and bottom waters results in heat and carbon (including that produced by human activity) is removed from the atmosphere. The carbon is in dissolved form in sinking water and the conversion of carbon into organic compounds. The ocean’s ‘high primary productivity’ https://tinyurl.com/y6aap3p6 is largely supported by nutrients being brought to the surface around Antarctica.

 Southern Ocean processing deep water

But the Southern Ocean is a ‘data desert’ he argues. Setting up and maintaining long-term, systematic monitoring of the Southern Ocean is ‘vital to our understanding of the global impacts of climate change in Antarctic waters.’

Mike Meredith is an oceanographer and Science Leader at the British Antarctic Survey (BAS) in Cambridge, UK. He is head of the Polar Oceans team at BAS, which has research foci on determining the role of the polar oceans on global climate, the ice sheets, and the interdisciplinary ocean system. 


The global importance of the Southern Ocean, Micheal Meredith, Ocean Challenge, 2019: https://www.challenger-society.org.uk/oceanchallenge/2019_23_2.pdf