A container with a Soviet submarine's nuclear reactor was discovered during survey work in the Kara Sea in northern Russia, authorities there said on Thursday.
Scientists are now due to investigate what was described as a "potentially dangerous object."
The reactor is said to have come from the Soviet nuclear-powered K-19 submarine.
Remains of military technology, including parts of a nuclear icebreaker fitted with a reactor, have been discovered in four bays of the Kara Sea in recent years, according to a report by Ria Novosti state news agency.
More than 1,200 radioactive fragments are still suspected to be under water.