What 1960 expedition made the first crewed descent to the Challenger Deep?

The 1960 expedition that made the first crewed descent to Challenger Deep was Project Nekton.

Project Nekton was a United States Navy programme of deep-sea test dives using the bathyscaphe Trieste. On 23 January 1960, Trieste carried Swiss oceanographer Jacques Piccard and US Navy Lieutenant Don Walsh to the floor of Challenger Deep in the Mariana Trench.

The descent took 4 hours and 48 minutes. The two explorers remained on the seabed for roughly 20 minutes before beginning their ascent, and the vessel returned safely to the surface. Instruments initially suggested a depth of 11,521 metres; later measurements revised the figure to about 10,916 metres.

Project Nekton is sometimes confused with later robotic missions such as Kaikō and Nereus, or with James Cameron’s 2012 solo dive in Deepsea Challenger. Those missions explored Challenger Deep later; Nekton was the expedition that first put people at the deepest known point in the oceans.