Files detailing the mysterious USOs – unidentified submersible objects – are kept tightly under lock and key at the Kremlin, according to UFO authors Paul Stonehill and Philip Mantle.

The pair claim to have obtained leaked details of some of the encounters between Russian submarines and giant unexplained objects.

They believe the Russian government has attempted to debunk any witness who went public to hide evidence of what was going on beneath the waves.

In new book, Russia's USO Secrets: Unidentified Submersible Objects in Russian and International Waters, the UFO experts suggest the encounters include meetings with underwater lifeforms, mysterious submersibles and even airbourn UFOs dumping mini crafts into the sea.