Even before the Iraqi military started its invasion of Mousl, the expectation was that the massive tunnel network ISIS was digging under the city would be a huge problem, giving ISIS access to a whole collection of routes totally unmapped by the invading forces.
It’s worse than anyone figured, however, with Iraqi troops finding that it’s not just Mosul itself with the tunnels under it, and even smaller villages in the surrounding area including multiple tunnels. Iraqi forces think they have them all and they come across another tunnel. Or worse, ISIS forces emerge from that tunnel and attack.
Iraqi Col. Falah al-Obaidi says the fighting is like trying to contest two cities at once, with a whole second underground city full of ISIS seemingly everywhere that the troops try to advance. With so many different tunnels, it’s hard to know if an area is ever really “secured.”
Heavy airstrikes by both Iraq and the US-led coalition have encouraged ISIS to dig a lot of tunnels to keep their movements and positions concealed, and it seems that this tactic is becoming a huge part of their defense of not just Mosul, but the surrounding area.