Tunneling techniques have been employed by hundreds of different guerilla and terrorist organizations throughout history, and the GLA are no different. Most notably, the GLA have expanded upon their vast tunnel network by modifying larger entrances/exits into forward operations bases, such as barracks, motor pools and even command centers. In a GLA base, whenever a surface structure is destroyed, the hole will remain. If it remains open, workers will eventually surface to rebuild the structure. It is very helpful to survive one blast from a superweapon for example like Nuclear Bomb or FOAB (but not from a Particle Cannon). From within the hole, a worker could emerge to rebuild the building in question, at no cost to the cost to the GLA commander.
The Worker that performed this task disappear back into the Hole (and presumably the GLA Tunnel Network) after construction. Enemy commanders had to be careful to eliminate the GLA Hole each time they destroyed a GLA structure, or the hard work of their troops would be undone. However, Holes are rather easily destroyed, not delaying a thorough attacker overmuch.