How Trieste Took Two People to the Bottom of Challenger Deep

On January 23, 1960, U.S. Navy lieutenant Don Walsh and Swiss engineer Jacques Piccard went down in a steel sphere suspended beneath a float. Their destination lay in the southern Mariana Trench, southwest of Guam. Five hours after leaving the surface, they reached the bottom of Challenger Deep. The dive marked the first human arrival at the deepest known part of the ocean. Everything about the vehicle had been arranged around one requirement: the pressure at that depth had to stay outside the small space occupied by the crew. The dive was part of the Navy’s Project Nekton, a series of increasingly deep tests before the descent in January. The bathyscaphe Trieste drew on work by Jacques Piccard’s father, Auguste Piccard, who had applied the idea of a light float and a separate pressure cabin to deep diving. From the surface, the long part of the vessel looks like the part that would carry people. The crew actually occupied the smaller sphere below it. A sphere distributes pressure evenly around its surface, and a thick metal wall can hold an air-filled space open against the surrounding water. The float above it provided the buoyancy needed to bring the heavy structure back up. Trieste’s float contained gasoline, which was less dense than seawater. The craft also carried iron shot as ballast. The combination gave the crew a way to control their movement without relying on engines to lift the vehicle back through eleven kilometers of water. Letting go of ballast made the bathyscaphe more buoyant. At depth, the pressure outside was so high that a cabin leak or a structural failure would leave no practical way to intervene from the surface. The method of returning was therefore as important to the design as the method of descent. The Navy’s oral history with Walsh gives a sense of the hours inside the sphere. The crew had instruments, a small viewing window and limited room. During the descent, they heard a loud report. A transparent outer layer of the entrance assembly had cracked under pressure, but the pressure sphere and its inner seal held. The incident is useful for understanding how the parts of the vehicle served different purposes. A failure in an exterior component could make a startling sound while the barrier that kept the crew alive remained intact. There was little time to explore once they arrived. Fine sediment stirred near the bottom and affected visibility. The Navy’s account records that the vehicle remained there for about twenty minutes before starting up. Reaching the seafloor demonstrated that a crewed craft could operate under full ocean pressure. It did not give the crew a wide view of the trench or establish the exact shape of every nearby depression. They had brought two people and a small set of instruments to one point on an extensive seafloor. The effort to measure the depth continued long after the dive. Challenger Deep contains several basins, with the bottoms separated by ridges and by many kilometers of horizontal distance. A ship can map the wider area with sonar by measuring the time it takes sound to return from the bottom. A submersible can measure pressure along a path close to the floor. Both methods require corrections, including for the properties of seawater. A 2021 study based on submersible transects reported 10,935 meters as the deepest seafloor depth it observed, with an uncertainty of six meters at the stated confidence level. That is a modern measurement and should be read separately from the depths recorded or reported during the 1960 dive. The map pin uses the coordinates of the modern survey’s deep point in the eastern basin. The 1960 dive established the historical milestone in Challenger Deep, but an underwater expedition from that period cannot be tied to this exact modern sounding merely because both took place in the same deep. Even a small difference between two coordinates can mean a long way at the bottom. The height of the water column also has to be stated relative to a surface reference before figures from different surveys can be compared. Walsh’s oral history is particularly useful because he had to think about the vehicle as an operator, not just as a person being carried in it. Project Nekton had included test dives before the record attempt. The team had to learn what the craft did as the pressure increased and how to tell which changes in its behavior called for a decision. An archival photo of Trieste before a dive and a photograph from a later descent record two ends of the same technical problem. At the surface, the float is visible and the crew can reach the vessel from a ship. Near the floor, the useful view is the small area that a submersible’s lights can reach. Between them lies a water column that must be crossed slowly enough to monitor the equipment and control buoyancy. The trip made the bottom of the Mariana Trench accessible for direct observation. The modern survey then answered a different question at finer resolution, tracing the shape and depth of the seafloor that Trieste had shown people could reach.