So, as Ulysses said in Lonesome Road, and as we saw as well there, the Tunnelers were able to live beneath the surface. They are hard, quick and hunt in groups. Those creatures would suppose a huge threat to any place in the world. Ulysses said they would take the Mojave easier than they took the Divide. Don't know why. They're not like Deathclaws, who can be saw and avoid them. When we enter that tunnel we have a direct comparison between Deathclaws and Tunnelers, and Obsidian was very clear saying that a unique Tunneler was able to slay a fucking Deathclaw as well. So even an army like NCR would have problems with those monsters. It is known, too, that the Marked Men used flare guns to scare them, but we haven't seen any of those in the Mojave (which doesn't mean they are unable to find there). As a quick information, a sole tunneler can survive 4 shots in the head with a full health Anti Materiel Rifle, approximately 112 points of damage for bullet. Opinions, guys?