Nereus

Nereus was a hybrid uncrewed autonomous underwater vehicle (HROV, a type of remotely operated underwater vehicle) built by the Woods Hole Oceanographic Institution (WHOI). Constructed as a research vehicle to operate at depths of up to , it was designed to explore Challenger Deep, the deepest surveyed point in the global ocean. Nereus, named for Greek sea titan Nereus (who has a man's torso and a fish-tail) through a nationwide contest of high school and college students, began its deep sea voyage to Challenger Deep in May 2009 and reached the bottom on May 31, 2009. On this dive the Nereus reached a depth of , making the Nereus the world's deepest-diving vehicle in operation at the time, and the first since 1998 to explore the Mariana Trench, the deepest known part of the ocean. On 10 May 2014, Nereus was lost while exploring the Kermadec Trench at a depth of . Communications were cut off at around 2 p.m. local time, and debris retrieved later revealed that it imploded due to high pressure. Hybrid design Being a hybrid ROV means that the vehicle could operate untethered or tethered with a thin optical fiber cable and operated by pilots aboard the ship. The latter enabled it to make deep dives while being highly maneuverable. The optical fiber tether has the approximate diameter of a human hair, and can bear only . It is made up of a thin layer of plastic surrounding a slender glass fiber core. The vehicle carried approximately of cable wound in two small canisters that pay out the fiber as the vehicle descends. This slim tether is smaller, lighter, and more cost-effective than cable.