Sinkholes appeared under Indiana sand dunes, as Gaea continued her war against the human race.
It's an intriguing approach, nearly Lovecraftian:
[S]and-covered trees... left cavities behind as they decayed over the years, researchers have found.
Fungi on the covered trees formed a sort of cement that allowed the sand to keep its hollowed out shape as the wood decayed and collapsed inward, leaving holes more than 10 feet (3 meters) deep...
(thanks to Steven Kaye)