I would
not be walking around that..
They have to be man caused. There aren't a bunch of these spattered all around. They all end up in South American cities, where pavement and plumbing run. And it's after decades of the city being there. I'm surprised to see no pipes or plumbing running thru the sides. Could they just sort of be running into cesspools that dissolve away the under-dirt?
Now the un-ded have direct access.