A cross bore occurs when a new natural gas line is installed using trenchless methods and intersects an existing underground lateral sewer connection. If not immediately identified, a gas-line cross bore can go undetected until it causes a backup in the sewer lateral. At that point, the resident will summon a plumber, whose attempts to clear the lateral may cause the gas line to rupture. When that happens, the escaping gas can accumulate in the adjoining residence and cause an explosion. This scenario can be avoided with a sewer lateral launch inspection wherever a new gas line is installed. According to the Gas Technology Institute there have been 18 accidents resulting from cross bores from 2002 to 2015—a number that the Cross Bores Safety Association says is more likely an understatement (source).