The first recorded sinking of a submarine by depth charge was "U-68", sunk by a Q-ship off Kerry, Ireland 22 March 1916. By early 1917, the Royal Navy had also developed indicator loops which consisted of long lengths of cables lain on the seabed to detect the magnetic field of submarines as they passed overhead. At this stage, they were used in conjunction with controlled mines which could be detonated from a shore station once a 'swing' had been detected on the indicator loop galvanometer. Indicator loops used with controlled mining were known as 'guard loops'. By July 1917, depth charges had developed to the extent that settings of between were possible. This design would remain mainly unchanged through the end of World War II. While dipping hydrophones appeared before war's end, the trials were abandoned.