Developed by engineers at the Woods Hole Oceanographic Institution, the $8 million, three-ton Nereus is the world’s first hybrid research vehicle: It is able to act both as an autonomous underwater vehicle (able to survey large regions with cameras and sonar ) and as a remotely operated vehicle (able to record images and collect samples on command). During its first test cruise in May, it dove 6.8 miles into the deepest part of the ocean, the Mariana Trench, a feat that only two other vessels have managed.
While in remote mode, a thin 25-mile-long fiber-optic tether transmits information between the Nereus and the research ship deploying it. Bright LEDs allow researchers to see about 10 feet ahead of the vehicle in deep waters where no sunlight penetrates. Nearly 1,500 softball-size hollow ceramic spheres  packed into the vehicle’s two hulls provide buoyancy and help it withstand up to 15,000 pounds per square inch of water pressure. If its tether breaks, Nereus can shift into autonomous mode, using its 2,000 rechargeable lithium-ion batteries  to navigate back to the surface. Nereus will allow scientists to explore the deepest parts of the seafloor, which had previously been inaccessible. There it will seek out new species and habitats; it may also study subduction zones, where oceanic crust is recycled back into the earth’s mantle.