July 26 1917, Cap Gris Nez–German submarines had, since the start of the year, once again been running the Dover Barrage, letting them attack Allied shipping in the English Channel without going all the way around Great Britain. Submarines doing so faced large risks, and on July 26, the minelaying submarine UC61 found that, for them, the risks were not worth it. In what may have been an attempt to skirt the barrage by sticking close to the French coast, the submarine ran aground on a sandbar. Attempts to refloat the submarine failed, and the crew set about destroying whatever they could.
These efforts were interrupted (and any thoughts of escape precluded) when the submarine was surrounded by a squadron of Belgian cavalry. The crew was captured, and Royal Navy personnel searched the ship. They found documents indicating that the Germans had cracked the code that the British used to report their minesweeping operations. Room 40 decided to use this to their advantage, broadcasting fake minesweeping information in that same code. This quickly paid off; on August 4, UC44 was sunk off of Ireland by mines laid by a different German submarine, which German naval intelligence falsely believed had already been swept by the British.