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.