The HMS Challenger: The Expedition that Discovered the Ocean

On December 21, 1872, a ship set sail that would change the way we view the ocean forever. The voyage of the HMS Challenger was the “nineteenth century British equivalent of the American space program,” describes Tony Koslow in his book, The Silent Deep. This expedition was among the first to inform the global understanding of the deep ocean, including its physical properties and diversity of life. A pinnacle of basic research – which is research that generates new knowledge about the world around us – the voyage of the HMS Challenger laid the foundation for nearly every modern branch of ocean science including marine geology, chemistry, geography, and ecology. 

Before the Expedition

Before the HMS Challenger set sail, the depths of the ocean were shrouded in mystery. Nobody knew exactly how deep the ocean was, what life existed in its depths, or even what the ocean floor was made of. This mystery fueled imagination, most famously seen in the novel 20,000 Leagues Under the Sea by Jules Verne, which was published in 1870, just two years before the Challenger expedition. This novel painted a fantastical picture of what life might look like beneath the waves. While the upper depths of the ocean had been accessed throughout history by fishermen and divers, what lay at the bottom of the ocean was largely unknown.  

This mystery also led to misconceptions. In 1841, Edward Forbes, a British naturalist, accompanied the HMS Beacon on an 18-month expedition to the Aegean Sea with the goal of learning about animal life in the Mediterranean. During this expedition, scientists used dredges to plow the seafloor for animal life. Forbes observed that the deeper they dredged, the fewer animals they found. This observation led him to form his (now disproven) “Azoic Hypothesis.”

The Azoic Hypothesis stated that ocean depths below 1,800 feet (550 m) were largely uninhabitable and devoid of life. “As we descend deeper and deeper in this region, its inhabitants become more and more modified, and fewer and fewer, indicating our approach towards an abyss where life is either extinguished, or exhibits but a few sparks to mark its lingering presence,” wrote Forbes in The Natural History of the European Seas in 1859.  From the limited knowledge of the deep sea at the time, it’s understandable how one could reach such a conclusion. After all, the ocean floor measures just a few degrees above freezing, endures crushing pressures, and lacks any sunlight with which plants can photosynthesize.  

This hypothesis, however, was not universally accepted, even at the time. Sir James Ross, the renowned polar explorer, frequently pulled up living animals with his sounding lines, sometimes from depths over 1,900 feet (585 m). In 1862, cable workers reported life from the North Atlantic much deeper than 1,800 feet (550 m), and in 1866, Georg Ossian Sars, a Norwegian naturalist, famously dredged a stalked crinoid off of Norway from a depth of 1,700 feet (820 m). To adequately test the Azoic Hypothesis, however, a scientific expedition was needed, the likes of which had never been undertaken.

The HMS Challenger Sets Sail

The expedition of the HMS Challenger was a massive undertaking. Funded by the British government, it would take four years to complete (1872-1876), would carry 237 men onboard, and would traverse almost every ocean on Earth. It had four main scientific objectives: (1) to investigate the physical conditions of the deep sea, (2) to determine the chemical composition of sea water across depth, (3) to understand the makeup of seafloor sediments, and (4) to examine the distribution of life throughout the ocean.   

To accomplish these goals, the Challenger set sail with six scientists onboard, including the chief scientist Sir Charles Wyville Thomson, a Scottish naturalist and marine zoologist. While he began his education studying medicine, he eventually turned his focus to the natural sciences and held many prestigious titles throughout his career, including secretary of the Royal Physical Society of Edinburgh in 1847, professor of botany at the University of Aberdeen in 1851, and the natural history chair at the University of Edinburgh in 1870. Thomson was also joined by (among others) Sir John Murray, a Scottish oceanographer who would go on to lead the writing of the expedition reports, and John James Wild, a Swiss linguist, oceanographer, and illustrator.

How They Collected Data

During the expedition, scientists used a number of different tools to gather data, many of which are still used today. To measure ocean depth, the HMS Challenger team used sounding lines and Hooke sounders. A sounding line is a weighted rope with measurement markings along its length (usually placed every 2 or 3 fathoms). Sailors lower the weighted end of the rope into the water until the weight hits the bottom (indicated by the line catching or going slack) and then take note of the length of rope they lowered. The Challenger also used Hooke sounders —a new invention at the time —where a glass ball was attached to a weight that would unhook from it when it hit the bottom. The scientists would then measure how long it took the ball to float back to surface, thus allowing them to calculate depth.  

To collect water, sediment, and biological specimens, dredges, trawl-nets, and tow-nets were used, as they still are today. Seafloor sediment was sifted through sieves to separate particles of different sizes, allowing scientists to study both the physical and chemical properties of the seafloor. Temperature readings of surface waters were also taken, giving scientists their first glimpse into global ocean circulation. 

The work was grueling. “From beginning to end the history of the Challenger Expedition is simply a record of continuous and diligent work,” writes Sir John Murray in one of his expedition reports (Summary of Scientific Results, Pt. 6, Vol. 1), “The labour connected with preserving, cataloguing, and packing the biological and other collections on board ship was enormous, so also was that involved in their subsequent examination on the return of the Expedition…” Indeed, once samples were brought back to land, they were distributed to naturalists and artists around Europe to help in identification, classification, and illustration. Working through so much data also took time, with the last expedition report published 19 years after the expedition concluded (in 1895). 

Major Findings

The HMS Challenger effectively jump-started the field of oceanography and was the first real attempt at unravelling the mystery of the deep ocean. It recorded data from 354 locations (stations) across all oceans except the Arctic. Findings were published across 50 volumes, totaling 29,500 pages. Over 3,000 lithographs, plates, charts, maps, and diagrams were produced, including the now-famous illustrations of Ernst Haeckel, a German naturalist, who depicted the beauty and diversity of ocean life collected during the expedition in stunning detail.

Such diversity of ocean life had never before been seen. The HMS Challenger discovered over 4,500 new species, and nearly 80 percent of all animals collected during the expedition were new to science. They also found that deep-sea species tended to have small geographic ranges, with about 40-46 percent of species collected from a single station.  

Also among the notable discoveries of the HMS Challenger was the discovery of the Challenger Deep, still the deepest known part of the ocean at the southern end of the Mariana Trench. While the Challenger’s original sounding measured its depth at 4,475 fathoms or 26,850 feet (8,184 m), modern echo-based soundings place the actual depth of the Challenger Deep closer to 6,011 fathoms or 36,066 feet (10,994 m). This sounding was so bafflingly deep at the time that scientists aboard the HMS Challenger took a second measurement, just to be sure!  

The Legacy of the Challenger Expedition

The work started by the HMS Challenger is still going strong today. As of 2023, there are now over 242,000 marine species known to science, with an average of 2,332 new species discovered and named each year. A huge portion of the over 50 million invertebrate specimens currently housed at the Smithsonian Museum of Natural History come from the ocean, with even more marine specimens held in natural history collections around the world. Remotely-operated vehicles and deep-sea submersibles like Alvin and the Johnson Sea Link have given researchers unprecedented access to the deep sea, and advances in telecommunications allow organizations like the Ocean Exploration Trust and the Schmidt Ocean Institute to livestream high-resolution videos straight from the seafloor.

One crucial departure from the legacy of the HMS Challenger is who gets to conduct deep-sea research. While scientists aboard the Challenger were all undoubtedly brilliant scientists, it is not a coincidence that they were also all men of European descent. Indigenous and non-Western knowledge of the ocean was overtly spurned by the Challenger science team, and women would not be allowed on research expeditions until nearly a century later. Today, deep-sea research is a global endeavor, with scientists from around the world regularly working, collaborating, and publishing work together.       

Finally, despite a century and a half of research since the HMS Challenger expedition, there is still a lot left to learn about the ocean! It is estimated that at last 2 million marine species still remain to be discovered, less than 30 percent of the ocean floor has been mapped to modern standards, and only about 0.001 percent of the deep ocean has been seen by human eyes. Filling these gaps is as important today as it was a century ago. In the words of Sir John Murray: “No complete theory of the earth [is] possible so long as we [are] ignorant of the conditions prevailing over the three-fifths of the globe covered by the waters of the ocean.”