KIRKUK, Iraq — There was a time not so long ago when the Islamic State (IS) was able to sweep through towns and villages with relative ease, expanding the boundaries of its self-declared caliphate miles at a time.
But as the various forces lined up against it have regrouped, gaining territory has become more difficult.
Now the militant group appears to be adapting its tactics. In many areas, it has taken the fight underground.
GlobalPost has obtained footage of a sophisticated tunnel in the village of Dur Karaba, near the city of Kirkuk, that Kurdish peshmerga commanders say was built by IS fighters.
The village was one of a string of towns held by IS southwest of the city until March, when the peshmerga led a double-edged offensive dubbed "Scorpion 1" and "Scorpion 2" that pushed them out.
Peshmerga fighters discovered the entrance to the tunnel in a house in Dur Karaba, just on the edge of the front line. When they went inside, they found that the tunnel reached for 160 meters — about 525 feet — underneath the no man’s land separating them.
Commander Kamal Kirkuki says the IS fighters were attempting to tunnel up behind the peshmerga lines.
“They were halfway there. It was a great danger. Who knows what would have happened if they had succeeded,” he says.
In the house where the tunnel began, Kirkuki and his men found peshmerga uniforms which they said IS fighters would have worn to cause confusion and chaos as they attacked.
The use of tunnels by guerrilla forces is nothing new. Germanic tribes used them more than 2,000 years ago against the Romans. Viet Cong rebels used a massive underground network, to devastating effect, against American troops during the Vietnam War.
More recently, in Syria, rebel fighters have used tunnels to break through static front lines in Aleppo and elsewhere to target government forces.
It appears the Islamic State is increasingly making use of tunnels in their areas of operation.
A recent report from NBC detailed how IS fighters dug a mile-long tunnel under a local army headquarters in Anbar. From it, they detonated 300 explosives and killed more than 40 Iraqi soldiers.
There have also been reports of the militant group using a vast underground network of tunnels in Fallujah and Raqqa, to move supplies and evade the US airstrikes that have targeted the group in Iraq and Syria.
“It makes it impossible for us to control this area," said an intelligence official south of Baghdad.
It is something that peshmerga soldiers are on the lookout for. Although they foiled this latest tunnel threat, they were not so lucky before.
Kirkuki recounts a story in which one tunnel did reach its intended length.
“There was nothing we could do. They killed so many of our soldiers.”