A Belgian company, Global Sea Mineral Resources, has been trialling a robot prototype, Patania II, in the Clarion-Clipperton Zone, which lies between Hawaii and Mexico.
The machine is meant to collect the potato-sized nodules rich in cobalt, and other metals used in batteries, that pepper the seabed in this area.
It was connected to a ship with a five-kilometre cable, but the company says it has come free and is now resting on the seafloor.
An operation to reconnect it is under way.