Interesting. Relevant quote from that link: "When a void occurs in sediment that has a certain amount of cohesion ('stickiness' among sediment grains), the most

Interesting. Relevant quote from that link: "When a void occurs in sediment that has a certain amount of cohesion ('stickiness' among sediment grains), the most stable configuration of the roof of the void is a dome, like the dome of the U.S. Capitol building. If that dome collapses, the vertical sides may remain upright, and the open hole will be circular."