There are a huge amount of caves, sometimes with difficult to find entrances, there's also underground areas to be accessed via wells.
I googled: You get it with finding relics.
It unlocks for activating a certain number of radio towers I think.
Speaking of which - how come I'll be standing on top of a relic symbol on my map, but I can't find any relics? I've been dreaming that there's some massive system of caves, but somehow I doubt it.