In January of 1960, Don Walsh and Jaques Piccard were the first to visit the deepest know point in the oceans. In the interest of time and convenience, I've included a general overview of the project below (from Wikipedia).
Trieste is a Swiss-designed, Italian-built deep-diving research bathyscaphe which reached a record depth of about 10,911 metres (35,797 ft) in the Challenger Deep of the Mariana Trench near Guam in the Pacific. On 23 January 1960, Jacques Piccard (son of the boat's designer Auguste Piccard) and US Navy Lieutenant Don Walsh achieved the goal of Project Nekton. It was the first manned vessel to reach the bottom of the Challenger Deep.