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 |
|Date of incident: |20 January 1921 |
|Shipping route: |Torbay to Spain |
|Passengers & cargo: |58 crew |
|Ran aground: |Sank 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.