Nereus was the autonomous underwater robot that reached the Challenger Deep in 2009.
Developed by the Woods Hole Oceanographic Institution, Nereus descended to the Challenger Deep in the Mariana Trench on 31 May 2009. The dive reached about 10,902 metres, making Nereus one of the few vehicles to operate at the deepest known part of the ocean.
Nereus could operate autonomously or be connected to a ship by a lightweight fibre-optic tether. Its hybrid design allowed it to survey the seafloor and carry instruments for scientific observation. The vehicle was built to withstand immense pressure at depths where ordinary equipment would fail.
Nereus is sometimes confused with Alvin, a crewed deep-sea submersible, or with Argo, an autonomous oceanographic float. Nereus was a robotic vehicle designed specifically for extreme deep-ocean exploration.