German U-boat U-534 mysteriously refused to surrender at the end of World War Two, and was on a northerly course from Denmark when it was attacked, and sunk, by an RAF Liberator. Most of the crew survived. The wreck was recovered in 1993 and now sits in Birkenhead. The ship itself has been cut away to reveal its rusted and destroyed interior. There is also an interesting exhibition.
Note - entry to U-534 might be free with a Mersey Ferry Explorer ticket. Managed by Mersey Travel.