U-864 was sunk by HMS/M Venturer off the Norwegian coast near Bergen in 1945 in what is believed to have been the only instance of one submerged submarine sinking another submerged enemy submarine.
In this gallery you can view some of the photographs taken when the ROV ( Remotely Operated Vehicle) recovered some of the mercury flasks from the wreck of U-864 and also the multibeam images taken that shows the wreck, as it is today, lying on the seabed. The full story can be read on the following link