The Zeebrugge Raid involved three outdated British cruisers chosen to serve as blockships in the German-held Belgian port of Bruges-Zeebrugge from which German U-boat operations threatened British shipping. "Thetis", "Intrepid" and "Iphigenia" were filled with concrete then sent to block a critical canal. Heavy defensive fire caused the "Thetis" to scuttle prematurely; the other two cruisers sank themselves successfully in the narrowest part of the canal. Within three days, however, the Germans had broken through the western bank of the canal to create a shallow detour for their submarines to move past the blockships at high tide.