As of 2021, the deepest location that a submarine has reached is the Challenger Deep, which is located in the Mariana Trench in the Pacific Ocean. The Challenger Deep is the deepest known part of the world’s oceans, with a depth of approximately 36,070 feet (10,994 meters).
The first manned submarine to reach the Challenger Deep was the Trieste, a deep-diving research bathyscaphe (submersible) built by the Swiss engineer Auguste Piccard and the Italian engineer and naval architect, designed by Trieste. It was first reached by Swiss engineer Jacques Piccard and US Navy Lieutenant Don Walsh on January 23, 1960. They reached a depth of 35,800 feet (10,911 m).
The second manned vessel to reach the Challenger Deep was the Deepsea Challenger, a deep-diving submersible built by the filmmaker James Cameron. On March 26, 2012, he reached the bottom of the Challenger Deep alone, and spent about three hours on the seafloor and collected samples and data.
The Mariana Trench is also known for its extreme pressure and cold, dark and hostile conditions, which makes it difficult for submarines to operate in that depth. Only a few manned and unmanned submarines have reached the Challenger Deep, and most of the research is done by using autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) that can withstand the extreme pressure and collect data and samples.