SOUTH OF MOSUL, IRAQ - The mouth of the tunnel is hardly visible on a muddy hillside overlooking Mosul, where fighting now rages between Iraqi forces and Islamic State militants.

In less turbulent times, trains ran through it on their way to or from Mosul, but when the militants overran the area in the 2014, they barricaded both ends, ripped up the tracks and built an assault course inside to train recruits.

Iraqi forces discovered the underground training camp after regaining control of the hillside last month.

Locals tipped them off about the location of the camp, which reveals the extent of extremists' determination, despite the overwhelming number and firepower of the forces arrayed against it, which are backed by a US-led coalition.

Clambering down a bank of earth that concealed the entrance, two Iraqi soldiers went into the tunnel - about 7 meters high by 5 meters wide, lighting the way with their mobile phones.

They illuminated IS slogans painted along the walls of the tunnel - around half a kilometer in length - and a series of obstacles, which one soldier tried out.

"Their training is similar to ours," said Kadhem al-Gharrawi, a member of the Rapid Response Division, an elite unit. "It's tough training for special forces."

It is not clear how many recruits passed through the camp or what became of them.

The physical drills complemented the group's ideological training, evidence of which is contained in booklets littering the floor of the tunnel, detailing its uncompromising doctrine.

A leaflet titled Types of Idolatry, lies beside empty cartons of orange juice drunk by the recruits and packaging of the boots and balaclava headgear they wore.

'Conquering Rome'

The railway was built in the early 20th century, as part of the line connecting Berlin to Baghdad. It was out of use when the IS overran Mosul in the summer of 2014 and declared a self-claimed caliphate spanning Iraq and Syria.

"By the will of God, we will conquer Rome," reads one mural painted on the wall.

Near the start of the assault course lie several backpacks full of sand, which were worn by recruits to weigh them down as they went over the obstacles, to increase the difficulty.

After coming off the death slide, recruits would have swung along monkey bars and then thrown themselves flat to crawl under barbed wire.

Red arrows point to the direction in which they were supposed to scramble over a wall - still covered in scuff marks made by their boots.

The recruits appear to have slept there: Bedding is strewed in two chambers dug into the sides of the tunnel, including a pink duvet cover decorated with cartoon character Mickey Mouse.