During an exercise ‘mock battle’ in the Bay of Biscay, K5 was unable to resurface from a dive. After a ditty box was recovered, it was concluded that she somehow exceeded her maximum diving depth.
- Job: Naval Fleet Submarine
- Shipping Company: British Royal Navy
- Launched: 1917, HM Dockyard, Portsmouth
- Tonnage: 2,565 tons
- Difficulty: Sank
- Date: 20 January 1921
- Shipping route: Torbay to Spain
- Passengers & cargo: 58 crew
- Location: Between Scilly and Ushant
Under Lt. Cdr. J. A. Gaimes, K5 sank with all hands lost: 5 Officers and 53 Ratings.
It is unknown what happened to prevent the submarine from resurfacing – possibly a technical fault.