Deep-sea exploration

• 1. Introduction

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  Deep-sea exploration sits at the edge of what humans have been able to reach. The Mariana Trench, the deepest point on Earth, was not visited by a person until 1960, which is more recent than the first satellite launch. And when Jacques Piccard and United States Navy Lieutenant Donald Walsh finally descended there in the bathyscaphe Trieste, they found fish. Life, at a depth of 10,915 m. That discovery upended a theory that had gone largely unchallenged for over a century.

  The ocean depths remain, as researchers describe them, a largely unexplored part of the Earth. The extreme cold and crushing pressure that characterize the deep sea have made it far harder to investigate than the surface of the moon. The questions this documentary follows are simple but stubborn: how did people figure out the ocean was that deep in the first place, what machines did it take to get there, and what did they find when they arrived?

• 2. The First Soundings

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  Pierre-Simon Laplace, a French scientist working in the late 18th or early 19th century, became one of the earliest people to attempt a scientific measurement of the ocean's average depth. He did not lower a rope into the water. Instead, he observed tidal motions recorded on the coasts of Brazil and Africa and used those readings to calculate that the Atlantic averaged 3,962 m deep. Later echo-sounding measurements would confirm the figure was quite accurate.

  The practical pressure to map the ocean floor came not from scientific curiosity alone. As demand grew for submarine telegraph cables, engineers needed accurate depth charts before laying them. Those commercial requirements pushed the first serious seafloor investigations. British explorer Sir James Clark Ross had already shown what instruments could do: in 1840 he lowered a sounding weight into the ocean and reached a depth of 3,700 m. The instrument was a simple weighted tube that drove itself into the seabed on impact.

  In 1818, another British researcher, Sir John Ross, pulled something unexpected from the deep. Catching jellyfish and worms in about 2,000 m of water, he independently confirmed that the deep sea is inhabited by life. That finding clashed with a prevailing idea that would harden into formal doctrine a generation later, when naturalist Edward Forbes declared in 1843 that life effectively ceased below 550 m, a claim he called the Abyssus theory. Michael Sars refuted it near Lofoten in 1850 by retrieving a rich variety of deep-sea fauna from 800 m. Then in 1864, Michael Sars and his son Georg Ossian Sars pulled a living stalked crinoid from 3,109 m off Norway, pushing the confirmed depth of life far past Forbes's limit.

• 3. HMS Challenger and the First Survey

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  From 1872 to 1876, HMS Challenger became the vessel that turned scattered observations into systematic science. A screw corvette converted into a survey ship in 1872, it covered 127,653 km under the direction of Charles Wyville Thomson. The scientists aboard collected hundreds of samples and hydrographic measurements and, by the expedition's end, had identified more than 4,700 new species of marine life.

  The Challenger crew worked with Baillie sounding machines, wire-line instruments that were an evolution of Ross's weighted tube, along with dredges and scoops suspended on ropes to scrape sediment and biological specimens from the seabed. Those physical samples gave the first real view of major seafloor features, including the deep ocean basins. Aboard the Challenger, researchers gathered data from all oceans except the Arctic.

  The Challenger expedition's findings on mineral deposits would echo far into the future. Polymetallic nodules discovered on the seafloor during the voyage, first noted in 1873, later became the foundation for commercial interest in deep-sea mining. Member states of the International Seabed Authority would eventually award 18 exploration contracts for the Clarion-Clipperton fracture zone of the Pacific Ocean, directly tracing their interest back to what the Challenger's scientists first pulled to the surface.

• 4. Into the Abyss: Bathysphere to Bathyscaphe

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  William Beebe was a naturalist from Columbia University in New York who recognized that observation from a lowered cage was not enough. Working with fellow engineer Otis Barton of Harvard University, he designed the bathysphere, a spherical steel pressure-resistant chamber attached to a cable. In 1930, the two men climbed inside and descended to 435 m. During the dive, Beebe peered out of a porthole and reported what he saw by telephone to Barton, who was on the surface monitoring from the ship. The arrangement had an obvious danger: if the cable broke, the occupants could not return.

  By 1934 the bathysphere had reached 923 m. Barton extended the record further in 1948, diving solo to 1,370 m. But the fundamental limitation of a tethered sphere was that it could not maneuver. The next design had to be self-propelled.

  Swiss physicist Auguste Piccard built that vessel. He called it the bathyscaphe, a navigable deep-sea craft with a gasoline-filled float and a suspended spherical gondola. On an experimental dive in the Cape Verde Islands in 1948, it held against the pressure at 1,402 m, though heavy waves afterward damaged its body. Piccard kept improving the design, and in 1954 reached 4,000 m. His son Jacques joined the effort in 1953, and together they built a new bathyscaphe that reached 3,139 m in field trials. The United States Navy acquired that vessel, named Trieste, in 1958 and fitted it with a new cabin rated for the deepest trenches. In 1960, Jacques Piccard and Lieutenant Donald Walsh took Trieste to the Challenger Deep at 10,915 m, the deepest known point on Earth, and observed fish and other organisms moving in the darkness around them.

• 5. Robots, ROVs, and Reaching Farther

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  The University of Southern California, working in the early 1950s with a grant from the Allan Hancock Foundation, built one of the first unmanned deep-sea vehicles. The device was a 3,000 lb steel sphere called a benthograph, carrying a camera and strobe light, designed to take photographs miles beneath the surface without any human inside. The original benthograph succeeded in capturing a series of underwater images before it became wedged between rocks and could not be retrieved.

  Remote operated vehicles, or ROVs, followed. Connected by cables to a surface ship, they can reach depths of up to 6,000 m and are guided in real time by operators above. A further development was the autonomous underwater vehicle, or AUV, which is programmed before launch and receives no instructions from the surface during its dive. A hybrid variant, the HROV, can operate either independently or via a cable depending on conditions. One unmanned vehicle was used in 1985 to locate the wreck of a famous sunken ship, with a smaller craft later entering the wreck to explore it.

  On the 25th of March 2012, filmmaker James Cameron made the second crewed voyage to the bottom of the Challenger Deep and the first solo mission. For the first time, the bottom was filmed and sampled by a single person. In 2018, Victor Vescovo piloted a vessel to the base of the Puerto Rico Trench, the deepest point in the Atlantic Ocean, at 8,375 m. In 2020, Dr. Kathryn Sullivan and Vanessa O'Brien joined Vescovo as mission specialists aboard the Limiting Factor and became the first women to reach the bottom of Challenger Deep, at 10,925 m.

• 6. Building Vessels That Survive the Pressure

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  At the depths reached by modern submersibles, external hydrostatic pressure becomes the governing fact of every design decision. The Deepsea Challenger's pilot sphere was constructed of steel and estimated to withstand 23,100 psi of hydrostatic pressure, roughly equivalent to an ocean depth of 52,000 feet, which is far deeper than Challenger Deep itself. The electronics of the same vessel were housed in smaller titanium spheres, because the smaller volume reduced the risk of catastrophic failure.

  Aluminum, steel, and titanium are the three metals most commonly used for high-pressure vessels. Aluminum suits medium-depth work where extreme strength is not the priority. Steel can be tuned for very high yield strength but its density limits how large a steel pressure vessel can be built without becoming too heavy. Titanium is nearly as strong as steel and about three times lighter, but it is more costly, harder to machine, and susceptible to flaws if processed incorrectly.

  The occupied portion of any crewed submersible must remain hollow and pressurized for human survival, while surrounding electronics can be protected inside syntactic foams or incompressible liquids. Maintaining internal pressure at roughly surface atmospheric levels simplifies life support considerably and removes the need for decompression on surfacing. All pressure vessel shapes, whether spherical, conical, or cylindrical, share the same rationale: distributing load evenly to minimize stress and prevent buckling. The Japan Agency for Marine-Earth Science and Technology, known as JAMSTEC, operates several AUVs whose construction reflects exactly this range of material tradeoffs.

• 7. What the Deep Revealed

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  In 1974, the submersible Alvin, the French bathyscaphe Archimede, and the French diving saucer CYANA worked together southwest of the Azores to explore the great rift valley of the Mid-Atlantic Ridge. About 5,200 photographs of the region were taken. Samples of young solidified magma collected on both sides of the central fissure provided direct evidence that the seafloor spreads there at roughly 2.5 cm per year, supporting the theory of plate tectonics.

  Between 1979 and 1980, scientists from France, Italy, Mexico, and the United States conducted a series of dives into the Galapagos rift off Ecuador. They found hydrothermal vents nearly 9 m high and about 3.7 m across, discharging hot water of up to 300 degrees Celsius mixed with dissolved metals in dark, smoke-like plumes. Those vents play a significant role in forming mineral deposits enriched in copper, nickel, cadmium, chromium, and uranium. Alvin also discovered giant tube worms on the Pacific Ocean floor near the Galapagos Islands on a separate research project, a find that reshaped understanding of where life can exist.

  Microbiological samples taken from the deep Tyrrhenian Sea during Mediterranean Science Commission expeditions confirmed the role of marine bacteria and viruses in deep-ocean productivity. They also established the contribution of autotrophic and ammonia-oxidizing Archaea to that ecosystem. Recovering sediment cores, which can reach back through geological time, allows scientists to identify fossils that indicate climate patterns during periods such as the ice ages. The drilling vessel JOIDES Resolution can extract cores from as deep as 1,500 m below the ocean bottom, giving researchers a direct archive of Earth history that no other tool can match.