The fight against the Islamic State terror group is heading underground. As Iraqi forces, with U.S. support, close in on Mosul, the toughest fighting likely will not be in the streets or door-to-door, but in sophisticated miles-long tunnel networks ISIL has constructed under the city. Fighting in this subterranean domain presents unique tactical, operational, technological and even legal challenges, for which the U.S. military must be better prepared. This is far from the first time tunnels have been used by asymmetric and conventional adversaries, and it is unlikely to be the last.
