The unmanned vehicle that reached the deepest part of the Mariana Trench in 2009 was Nereus. Built by the Woods Hole Oceanographic Institution, Nereus was a hybrid vehicle that could operate either autonomously or under remote control through a very thin fiber-optic tether.
On May 31, 2009, Nereus descended to Challenger Deep, the deepest known location in the world’s oceans. It reached approximately 10,902 meters and spent more than 10 hours operating over the seafloor, sending live video and collecting liquid and rock samples. The dive made Nereus the deepest-diving vehicle then in operation and the first vehicle to explore the Mariana Trench since 1998.
Several vehicles are commonly mixed up with Nereus. The crewed bathyscaphe Trieste first reached Challenger Deep in 1960, while Japan’s unmanned Kaikō made an earlier robotic descent in 1995. Deepsea Challenger was the vehicle used by James Cameron for a solo crewed dive in 2012. Argo, meanwhile, was an unmanned camera sled used during earlier expeditions rather than the 2009 Challenger Deep descent.
Nereus was eventually lost in 2014 while exploring the Kermadec Trench.