I partially agree with Baker here.
Not FE.
While hollow earth is a large stretch, it's fundamentally on the right track. It's far more likely this is a result of a 'honeycomb' style of sub-terrainian structures beneath the surface. Think like how ants have tunnels and nests.
If you actually do some digging there is a lot of support for sub surface cave systems under the surface, look at recent revelations re: Antarctica and it's massive underground lake, and see where that bit of Intel leads you.