was not designed for such depths. Lewis made depth determinations by dropping explosive charges over the side and timing the interval between the explosion and the return echo. Over 300 explosive charges were used to locate the target trench area 4 mi long and 1 mi wide.

On dive 65 (eighth in the Nekton series), on 23 January 1960, Trieste reached the ocean floor in the Challenger Deep (the deepest southern part of the Mariana Trench), carrying Jacques Piccard (son of the boat's designer Auguste Piccard)