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They aren't that common, but they aren't that uncommon either, and they have various causes, like manmade caverns (like mines), natural caverns etc where it's pretty obvious how they can swallow up the soil when they collapse, There was one like this nearby where I live, and it was caused by a broken water main, the water running out of the water pipe washed all the soil away into the sewer system.
Originally posted by aRogue
So how do these things work?
Where did all the soil dissappear to?