We need to do some serious testing on these people. They are like moles. No matter where they are they dig tunnels.
On a serious note, you're right - they like hiding in tunnels and caves. Why? Because doing so has preserved them from foreign invaders for centuries. Most recently, it was highly effective against the Soviets. It worked well against the British and the French, too.
Unfortunately for them, the US learned about tunnels the hard way in Vietnam (though in the end we beat everyone who thought it was a good idea to hide in a tunnel complex) and have spent the last 35 years devising new and more efficient ways to detect and eliminate people who like hiding in holes or digging tunnels.
Pretty much there's nothing they can do to keep from getting caught or slaughtered by the US. And that's a good thing.