For a long time it would be easy for diving professionals and other experienced divers to distinguish a sport diver with an open-circuit scuba such as an aqualung from a combat frogman with a rebreather; and legitimate civilian divers are normally fairly easy to detect because they dive from land or from a surface boat, rarely or never from an underwater craft, and willingly advertise their presence for their own safety; but recent multiplication in sport rebreather use may have changed that somewhat.