Transported to a new base at San Diego, she was extensively modified and then used in a series of deep-submergence tests in the Pacific Ocean during the next few years, including a dive to the deepest known part of the ocean in January 1960. In late 1959 Trieste had been fitted with a stronger pressure sphere and was transported to Guam by the freighter Santa Maria to participate in Project Nekton - a series of very deep dives in the Mariana Trench. On January 23 1960, she reached a record depth of 35,813 feet in the Challenger Deep carrying Lieutenant Don Walsh, USN, and Jacques Piccard (son of Auguste). The descent took almost five hours and the two men spent barely twenty minutes on the ocean floor before undertaking the 3 hour 15 minute ascent.
In April 1963, she was modified and used in the Atlantic Ocean to search for the missing submarine USS Thresher (SSN-593). In August 1963, Trieste found the wreck off New England, 8400 feet below the surface. The bathyscaphe was then retired and some of her components were used in the Trieste II.
Trieste is a permanent exhibit at the Navy Museum in Washington, DC.
In appearance at the time of Project Nekton she was over 50 feet long, but the great extent of this was a series of floats filled with 22,500 gallons of gasoline to provide buoyancy and air tanks at either end of the ship. The crew were in a six foot diameter steel sphere attached to the underside of the floats. An additional nine tons of pellet shot were taken on the craft to speed the descent.