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.

Incident * 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.