Yesterday, the United States dropped the “mother of all bombs” on a network of Islamic State caves and tunnels in Afghanistan. The military said its spectacular use of force was necessary to destroy the underground terror hideout (though it's not clear that the MOAB was the right tool to accomplish that goal.)

Tunnels are not just the purview of the Islamic State's small Afghanistan force. Like other guerrilla groups, the Islamic State has created tunnel systems underneath many of the cities and villages that they occupy. These pathways are essential to their strategy, enabling them to move stealthily, strike quickly and escape capture.

It's hard to know how many tunnels exist or where. But anecdotal reports suggest that the network is extensive. After Iraq troops began trying to take back Mosul in 2016, for example, Iraqi troops and the Kurdish peshmerga found that the road into the city had been honeycombed with tunnels, many booby-trapped. Mosul, too, had an extensive underlayer of pathways. As an Iraqi intelligence officer told my colleague last year, “they're everywhere.” The same was true underneath Fallujah.

Iraqi forces say the tunnels have made an already difficult offensive harder, allowing Islamic State fighters to appear seemingly out of nowhere. This infrastructure allowed Islamic State fighters to creep quickly into position, then ambush advance troops from concealed locations. One commander recounted driving a tank into Eastern Mosul. He watched as dozens of Islamic State fighters quickly slipped from the street to the tunnels. Sometimes, the fighters would pop out after a neighborhood had been secured, firing on civilians and troops.

Rukmini Callimachi, who covers the Islamic State for the New York Times, explained the strategic advantage on NPR:

“In all of the areas that I have visited, ISIS dug a complicated network of tunnels. And so what they're able to do is they retreat inside the tunnels. And then from there, they're able to send a drone up into the air. So they're completely protected and unseen from our surveillance. And yet, they're able to see. So what they suspect is happening is that the drone is sent out to collect information to identify the location of where the enemy troops are. And then from there, they're able to pinpoint that place and then start aiming mortars at it as well as aiming munitions from the drone itself.”

“It’s like we are fighting two wars in two cities,” Col. Falah al-Obaidi of the Iraqi counterterror forces told The Washington Post. “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 a long military history. As my colleague William Booth explained:

ISIS didn’t invent the tactic. Tunnels have been used in warfare for thousands of years, especially valued in asymmetric guerrilla war. Jewish rebels used tunnels against Roman legions; the Viet Cong did the same against U.S. troops in Southeast Asia.
Here in Karemlash, Iraqi Christian militias uncovered cramped earthen burrows used by ISIS fighters to hide from surveillance drones, artillery shells and U.S.-led airstrikes.
The tunnels were dug into a hill that covers an archaeological site for an ancient Assyrian city. The Islamic State also commandeered the Saint Barbara convent and dug deep tunnels through the floor of the chapel. The village marks the site where Alexander the Great fought the Persian emperor Darius in 331 B.C.

In Iraq, the Islamic State co-opted tools used to help with mining or in the oil fields. The organization's fighters probably dug most of the network, aided by enslaved civilians. To hide their digging from drones and satellites, the Islamic State would hide the dirt inside nearby houses. Many tunnels are lit with electric lights; in some, Iraqi forces found dormitories, flowered wall paper and makeshift kitchens.

The network of tunnels illuminates something else: that it will be nearly impossible to bomb the Islamic State into submission.

“It's absolutely impossible,” Callimachi told Terry Gross. “Unless you're willing to court, you know, a real human catastrophe, you can't just bomb this place indiscriminately.”