UB-21 (Walther) carried out what appears to have been intended as a sabotage mission at Ness Point, Hayburn Wyke, North Yorkshire on July 19, 1917. The mission is sketched in PG61764. A two man party was landed from the boat. Their mission was apparently to blow up a railway line. The mission failed. Steuermann Gallus and Bootmsansmaat Sohlmann were captured by the British on July 29. Admiralty intelligence believed their story that they had been on the conning tower of the boat when it was forced to make an emergency dive, leaving them stranded