HMS Seahorse (98S) was a group one British S class

British S class submarine (1931)

The S-class submarines of the Royal Navy were originally designed and built during the modernisation of the submarine force in the early 1930s to meet the need for smaller boats to patrol the restricted waters of the North Sea and the Mediterranean Sea replacing the British H class submarines...

 submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 that was lost at sea with all hands in Heligoland Bight, North Sea, probably on the 7 January 1940, during the Second World War.

On 26 December 1939 she sailed from Blyth for patrol off the east coast of Denmark. Four days later she shifted position to the entrance of the Elbe. She did not return on her due date of 9 January 1940. It was first thought that she had been mined but German records, examined after the war, suggest she was the victim of the German First Minesweeper Flotilla which reported a sustained depth charge attack on an unidentified submarine on 7 January 1940. It is however also possible that she was rammed and sunk by the German Sperrbrecher IV/Oakland south east of Helgoland on 29 December 1939.