ISIS tunnels: How ISIS militants evade airstrikes in Ramadi

Despite this setback, ISIS still controls as much as 25% of Ramadi, local tribal leaders say. And fighting continues to rage in pockets throughout the city.

So how does ISIS evade airstrikes?

In Ramadi, as in many ISIS strongholds, the answer lies below the ground.

"During our advance to cleanse the area, they would distract us and disappear," Maj. Gen. Sami Kathim, commander of Iraqi Counter Terror Services, told CNN.

Part of a special operations unit, Kathim's team was among those tasked with hunting the militants. "We discovered they were ducking down and hiding into the tunnels."

The tunneling isn't exactly a feat of engineering, but it's a priority in ISIS towns. First the militants take territory, then they dig it up.

At about 10 meters (more than 30 feet) below ground, the tunnels are no more than a meter or 2 wide (3.2 to 6.5 feet).

"They are dug between houses so that they can cross the street without the planes seeing them. They go down and split off into separate tunnels. This tunnel we're looking at here goes on for a kilometer," Kathim said, pointing to one of the passageways. "Some others are 700, 800 meters."

What lies beneath the ground is proving critical to battles waged from above. The soldiers have already found a high-ranking ISIS commander hiding in the network of tunnels under Ramadi's streets.

"The areas that they are preparing to entrench in and defend, they make sure all the tunnels are interconnected," a soldier told CNN. "There is even electricity running inside the tunnels."

Makeshift shelters hide the movements of militants where the tunnels reach the surface.

And clearing the tunnels can be painstakingly slow as well as life-threatening. The troops have to ensure they are not rigged with explosives or booby-trapped.

The first man to venture inside is "carrying the lives of all of his comrades with him," Kathim said.

(CNN)