Divers searching the Undersee Hafen had reported a sighting of what appeared to be a new form of mine and, on the 8 May, Gosse dived and verified that it was a G.D. pressure mine known as the 'oyster'. Gosse was given the task of recovering the device intact and dived the next day to remove the primer, with improvised tools, rendering the device safe. During this procedure he heard a loud noise which, upon examination after the mine was brought to land, was found to have been caused by the detonator firing as the primer was removed. He later defused two similar mines which were posing a danger to shipping, on both occasions the detonator fired before the mine reached the surface.