Photos take us inside ISIS' underground lair

REUTERS/Ari Jalal

Despite continuous US-led coalition airstrikes and on the ground offenses by Kurds, Shiite militias, and the Iraqi government, ISIS has proven remarkably resilient at holding territory. 

In large part, this is due to the militant's habit of digging tunnels throughout territory that they control. These warrens allow ISIS to take shelter against airstrikes while allowing the group to move at least somewhat freely throughout territory they control. 

Such tunnel systems have been found across Iraq in territory that ISIS once held before being pushed out. Below are photos from one of ISIS's underground lairs in Sinjar, Iraq.

A member of the Kurdish Peshmerga forces inspects a tunnel used by Islamic State militants in the town of Sinjar.

The tunnels connected houses throughout the town, allowing ISIS fighters to move freely and avoid airstrikes.

Building tunnels is one of ISIS's first goals after taking over new territory.