Looking for an underground leak in a propane or natural gas line? Go back to nature. In addition to using your nose to detect the smell of the odorant (usually a mercaptan or similar compound), it is not unusual to see flies buzzing around the point where gas is leaking out of the ground. Odorants are similar to the chemicals given off by rotting food, and the flies can’t tell the difference. Other signs are dead grass or vegetation. If the ground is wet, you may also see bubbles.
(Sources: R.Meier; and Explosion Investigation and Analysis by P.Kennedy)