The article says it's karst. (Sinkholes occur when an already-significant underground cavity, typically formed by water dissolving the rock, gets close to the

The article says it's karst. (Sinkholes occur when an already-significant underground cavity, typically formed by water dissolving the rock, gets close to the surface and the roof falls in.) https://en.wikipedia.org/wiki/Sinkhole https://en.wikipedia.org/wiki/Sinkhole