HMS Challenger: How a 150-year-old expedition still influences scientific discoveries today

Over a century ago, one of the most important scientific expeditions in history departed from the UK to explore the world's oceans.

When HMS Challenger left Portsmouth on 21 December 1872, the crew of scientists and sailors aboard had little idea what they would discover over the next four years. Though some would depart the expedition early, and others would never return, their voyage would discover thousands of species, locate the deepest trench on Earth, and even contribute to our knowledge of space.

As the Challenger expedition approaches its 150th anniversary, scientists are continuing to make new discoveries from the specimens collected by these Victorian pioneers. 

Dr Stephen Stukins, Senior Curator of Micropalaeontology at the Museum, says, 'Challenger 's voyage of discovery 150 years ago was very impressive for its time.'

'They had a lot of foresight, such as keeping ridiculous amounts of rope on board to measure the deepest parts of the ocean and collecting samples that we only began to unlock the science of over a century later.'

'I couldn't imagine an expedition being so well-funded today and taking such a holistic view of the oceans.'

In recognition of the expedition's ongoing importance, the Museum will be hosting the Challenger Society’s biennial conference on this 150^th anniversary, gathering scientists from across the UK and the world to discuss the latest research in oceanography – an entire scientific discipline kicked off by the voyage.

From England to South Africa

At the start of the 1800s, the existence of life in the deep sea was a hotly contested area of debate. The French naturalist François Péron believed the seabed was permanently covered in ice, while Edward Forbes suggested the water pressure below 600 metres made life impossible. 

However, new methods for measuring the depth of the ocean and retrieving samples, known as sounding, showed that life could exist at great depths. This prompted the Royal Society to lobby the UK government for a global scientific expedition that would study all aspects of the oceans.

In response, HMS Challenger was provided by the Royal Navy in 1871. Most of the guns were removed to be replaced by scientific laboratories, deep-sea sampling equipment and dredging ropes over seven kilometres long.

After leaving for Gibraltar in 1872, the expedition got off to a few false starts as the crew accidentally broke some of the ship's dredging lines. However, they soon discovered one of their first new species - Umbellula thomsoni, a bioluminescent cnidarian related to jellyfish.

Throughout the voyage, over 4,700 new species would be described, including around 10% of all known starfish. As the voyage zigzagged across the biodiverse waters of the Atlantic on its journey south, many new species were collected such as the rare brachiopod Abyssothyris wyvillei.

This animal was found more than 4,800 metres below the surface, much deeper than scientists had expected, and was described as 'one of the three most interesting specimens' collected by the voyage.

This section of the voyage also saw many discoveries of plankton known as Radiolaria, of which over 2,000 new species would be discovered over the course of the voyage. Some of the species were named after the crew of the ship in the genera Challengeron and Protocystis (formerly Challengeria).

Another group recorded in abundance were foraminifera, which use the alkaline mineral calcium carbonate to make their shells. Now, 150 years later, these same specimens have been used to investigate the impact of climate change on the oceans.

'At the time, the importance of these specimens to study the climate weren't appreciated,' Stephen explains. 'It wasn't until the 1960s that Sir Nicholas Shackleton would link foraminifera to ancient climate and ice ages, which have since been built on to investigate more recent climate change.'

'I was part of a study which compared plankton tow material from Challenger to that collected by a recent expedition. While the species composition was mostly the same between the expeditions, we found the thickness of their shells has thinned over the past 150 years.'

'It's not just one species, but every specimen and species we looked at. This thinning is due to ocean acidification as a result of increased carbon dioxide in the atmosphere.' 

Exploration of the Southern Hemisphere and Australasia

Just over a year into its voyage, HMS Challenger left South Africa to journey across the Antarctic circle towards Australia and New Zealand.

Along the way, the ship visited and worked on isolated islands including Marion, Kerguelan and McDonald. The crew discovered new islands, as well as a range of unique species such as a moss now known as Dicranoweisia insularis. 

The next port of call for Challenger was Australia in March 1874, where the scientists received specimens of the thylacine, otherwise known as the Tasmanian tiger. With the last known individual dying in 1936, the anatomical studies conducted of these animals are still some of the most detailed of this extinct species.

Between being entertained by the country's officials, the scientists journeyed inland to investigate more of the continent's flora and fauna. Some of their methods, such as dynamite fishing, would never be used by modern scientists.

Challenger's time throughout Australasia is also marred by their attitudes towards the area's Indigenous peoples. While there was some collaboration with Indigenous communities on collecting, not all of the crew's interactions were as positive. Accounts of the voyage also use outdated language and ideas that are considered racist and unacceptable.

After visiting New Zealand, the expedition headed past Fiji and up to Hong Kong, where the ship's captain, Captain Nares, was replaced by Captain Thomson on 2 January 1875. The new commanding officer took the ship past the Philippines, where the expedition surveyed the newly formed volcano Mount Vulcan.

The expedition then headed down to New Guinea and the Admiralty Islands, but as this took longer than expected Challenger was forced to abandon attempts to visit Pacific islands such as Guam and instead head straight for Japan.

This proved fortunate, allowing the expedition to make a landmark discovery. While taking a depth sounding, Challenger discovered the Mariana trench, the deepest point on Earth, and recorded a depth of over 8,200 metres. 

More modern measurements put the trench's lowest point, named Challenger Deep in honour of the expedition, at roughly 11,000 metres. Many of the ship's soundings still form the basis of seabed maps today, with its findings comprising the basis of modern oceanography.

The voyage home

Following a stopover in Japan for repairs in April 1875, HMS Challenger set out across the Pacific Ocean for Hawaii. While dredging the ocean floor, they began to collect lumps of metal around the size of large potatoes known as polymetallic nodules.

Museum scientist Professor Richard Herrington says, 'These were the first polymetallic nodules ever discovered, and as they were found in a few locations across the Pacific, this made them even more interesting to scientists.'

'The nodules were cut open, and it was recognised they had nucleated around shark teeth and even a sponge. While the nodules were appreciated for their scientific curiosity, their preservation of animal remains was also of interest.'

'While their mineral value was thought to be limited at the time, they are today potential targets for seabed mining.'

On arriving in Hawaii, the Challenger expedition also noted another modern concern - the issue of invasive species:

'American plants are covering the ground, and American birds have been introduced and bid fair to spread and oust the native fauna, which has no single land bird in common with any other Polynesian island group.'

Since this account was written, invasive species introduced by both Americans and Europeans have contributed to a wave of extinctions across Hawaii. The Hawaiian Biological Survey estimates at least 271 species have gone extinct in the past 200 years, with the US proposing nine more species from the island be considered extinct last year.

The disappearance of wildlife was not the only loss Challenger witnessed in the Pacific. One of the expedition's scientists, Dr Rudolf von Willemoes Suhm, died of an infection on route to Tahiti. In his honour, a genus of crustaceans, Willemoesia, as was Suhm island near Kerguelen.

However, it wasn't all bad news - some of the voyage's Pacific discoveries were out of this world.

'Challenger is important because it's the first time micrometeorites were ever collected,' says Dr Natasha Almeida, the Museum's meteorite curator. 'These objects are below a millimetre in size and come from several sources, such as comets and asteroids.'

'It's remarkable they had the foresight to consider micrometeorites would collect at the bottom of the ocean and they could separate them from dredged material using magnets. As far as I'm aware, it's the oldest collection of micrometeorites in the world by at least 100 years, if not more.'

After sailing down the coast of South America, the ship crossed back across the Atlantic to Ascension Island and then back to England. After four years, the voyage had returned home. 

The legacy of Challenger

Across Challenger's long voyage, specimens had been periodically packed up and sent back to the UK, where they were stored at the University of Edinburgh. Remarkably, only three bottles arrived broken from among thousands of samples shipped halfway around the world.

The specimens were then sent to the leading scientists of the day in their field, with extensive reports compiled over the next decade. Eventually, the type specimens, which are the named representatives of a species, were sent to the Museum while duplicate specimens were split across other institutions.

Even 150 years later, these specimens are still important for research. Stephen, for instance, is hoping to make use of samples from other collections to assess how foraminifera are being affected by climate change in greater detail.

'Comparing Challenger material to that from other expeditions, such as the Discovery cruise, would allow us to map these changes in more detail, and may allow us to predict how these species could change in the future,' Stephen says. 'This could also involve taking a closer look at how acidification has changed in the past 150 years.'

The collections are also continuing to reveal new specimens, with Natasha part of a team of scientists extracting more micrometeorites from the ocean bottom deposit collection.

'At the time, micrometeorites were very difficult to study, but the development of new technologies allows us to do much more with them,' Natasha explains. 'Extracting new samples can allow us to compare how the number of micrometeorites hitting each part of the world varies.'

'We're hoping to compare these new micrometeorites from the Challenger material with modern specimens from Antarctica which are more pristine because it's so dry there. We hope this will allow us to come up with a weathering scale for ocean-collected micrometeorites.'

'Researching such small samples can also help us prepare to study small samples from asteroid return missions, such as Hayabusa2 and OSIRIS-REx.'

The samples are also informing important decisions that will shape the future of our planet's ecosystems and our economies.

'Since the 1870s, the polymetallic nodules collected by Challenger have been appreciated as a possible source of rare metals,' Richard says. 'They act as the type specimens of these structures.'

'There are real challenges to extract the necessary metals for a green revolution from the resources found on land, so minerals on the seabed are an obvious place to investigate.

'However, we still don't have enough information about what the effects of seabed mining will be which is why Museum scientists are continuing to investigate the biodiversity of the nodule fields.'

'Challenger specimens are part of the data being used to consider the benefits, as well as the impacts, of any mining operations.'

As the deep sea continues to be explored, more scientific discoveries will be made that tell us more about our world's past, present and future. Much of it will continue the work of HMS Challenger, the impact of which continues to shape scientific thought today.