“When U.S. and Iraqi troops invade the Islamic State group’s stronghold in Mosul, a big part of the battle may occur underground. The massive military operation, expected to begin in October, will be complicated by a network of tunnels beneath the city. These labyrinths will offer a key tactical advantage to the estimated 4,000 to 10,000 ISIS fighters who are entrenched in Mosul. “
Source: Tunnel warfare: In Mosul, the ISIS fight is likely to go underground