WOODS HOLE, Mass. (AP) — A remote-controlled research sub exploring some of the deepest depths of the Pacific Ocean has been lost.

The Woods Hole Oceanographic Institution says the Hybrid Remotely Operated Vehicle Nereus was lost during its six-mile dive to the Kermadec Trench on Friday. The sub was being operated as part of the Hadal Ecosystems Studies (HADES) Program funded by the National Science Foundation.

The hadal zone is the name given to the deepest depths of the ocean, named for the underworld of Greek mythology. The Kermadec Trench is the fifth-deepest in the world, about 75 miles off the coast of New Zealand.

Woods Hole science editor Ken Kostel called the loss of Nereus a “body blow” but the research team will continue their work to learn more about “a place that does not give up its secrets easily.”

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