WASHINGTON • When the United States dropped the "mother of all bombs" on a network of Islamic State in Iraq and Syria (ISIS) caves and tunnels in Afghanistan last week, the military argued that the move was necessary to destroy the underground terror hideout.
Like other guerilla groups, ISIS has created tunnel systems underneath many of the cities and villages that they occupy. The systems are essential to their strategy, enabling them to move stealthily, strike quickly, and escape capture.
It is hard to know how many tunnels exist, or where. But reports suggest the network is extensive.
After Mosul was liberated in 2015, for example, it was found that the road into the city had been honeycombed with tunnels, many booby-trapped. Mosul, too, had an extensive under-layer of pathways.
Iraqi forces say the tunnels have made an already difficult offensive harder, allowing ISIS fighters to appear seemingly out of nowhere. They allow the fighters to creep quickly into position, then ambush advance troops.
There's the war on the streets and there is a whole city underground where they are hiding. Now it's hard to consider an area liberated, because though we control the surface, ISIS will appear from under the ground, like rats.
COLONEL FALAH AL-OBAIDI, of the Iraqi counter-terror forces.
One commander recounted dozens of ISIS fighters in eastern Mosul slipping from the street to the tunnels. Sometimes, fighters would pop out after a neighbourhood had been secured, firing on civilians and troops.
"It's like we are fighting two wars in two cities," said Colonel Falah Al-Obaidi of the Iraqi counter-terror forces. "There's the war on the streets and there is a whole city underground where they are hiding. Now it's hard to consider an area liberated, because though we control the surface, ISIS will appear from under the ground, like rats."
Tunnels have been used in warfare for thousands of years, especially in asymmetric guerilla wars. Jewish rebels used tunnels against Roman legions; the Viet Cong did the same against US troops.
In Iraq, ISIS fighters probably dug most of the network, aided by enslaved civilians. To hide their digging from drones and satellites, ISIS would hide the dirt inside nearby houses. Many tunnels are lit with electric lights; in some, Iraqi forces found dormitories, flowered wallpaper and makeshift kitchens.