'Monstrous' was a US 'swamp' operation undertaken by US Navy destroyers to locate and sink Oberleutnant Siegfried Koitschka’s U-616 in the Mediterranean area to the west of Oran in Algeria (14/17 May 1944).

The crew scuttled its boat on 17 May in the area to the east of Cartagena in Spain after it had sustained fatal damage from the depth charges of the destroyers Nields, Gleaves, Ellyson, Macomb, Hambleton, Rodman and Emmons, and the depth charges from a Vickers Wellington maritime patrol bomber of the British No. 36 Squadron, in the course of an action which lasted for three days. The U-boat’s whole 53-man crew survived.