Air pocket or something like that then the land above giving way. I may be wrong, search wikipedia, they always have the answer.
IIRC it's caused by running water eroding ground underneath the surface. However, I've never seen them as symettrical as thing one... pretty insane.
Its something of that nature, like underground water removes some of the ground and leaves an air pocket type of thing and it collapses at some point.
like geology? Maybe my professor will go through this during the lectures. Seems scarey! Hopefully it's not common