Amid the wreckage of the town of Sinjar in Iraq, Kurdish peshmerga forces, which recaptured the town from Islamic State (Isis/Daesh) in November, have discovered a network of tunnels buried deep underground. Hidden from sight beneath the streets and shattered buildings, IS militants dug the tunnels, wired with electricity and fortified with sandbags, to protect themselves from coalition air strikes and prepare defences against the Kurdish onslaught.

"Months ago, we received information from reliable sources indicating they [IS militants] brought 700 prisoners and people from the villages of Tal Afar and other villages for 30,000 Iraqi dinars (£18, $27) per day," Wais Faiq, the head of Sinjar town council said.

"They dug so many [tunnels] to the extent that they completely destroyed the infrastructure of Sinjar. There is a tunnel under every alley, street and public building that remains intact. This is clear evidence the terrorist Daesh group is aware of Sinjar's geographical importance as it can link Raqqa to Iraq and for that reason they kept a tight grip on Sinjar."

Faiq said more than 70 tunnels had been discovered, many containing ammunition boxes, medicines and bombs. In one tunnel, more than 90 bombs were found. Kurdish forces retook Sinjar town from the Sunni militant group in a two-day offensive backed by air strikes from a US-led coalition. Evidence of the horrors that took place there is now being unearthed, with a grave containing the remains of more than 70 older Yazidi women among the discoveries. The excavated network of underground tunnels enabled the militants to move around the town undetected.

"The tunnels are not only in Sinjar, but in any place they take. They use them to hide from coalition air strikes. Their defensive plan is to go to these tunnels when the coalition drones come in," Sinjar's deputy operations commander, General Sami Mulla Mohammed Bosli, said.

Before it was overrun by IS, Sinjar and the surrounding villages were home to about 200,000 people, mainly Kurdish and Arab Muslims – both Sunni and Shi'ite – as well as Christians and Yazidis, a faith that combines elements of several ancient Middle Eastern religions. Now, following all the fighting, the town is largely deserted.