I'm watching a documentary type show about sinkholes. We have had them for years in Florida. Scientists suggest it's because of the heavy layer of limestone, which is a soft mineral, that depletes over time due to water, and it opens up sinkholes. From watching this, it seems it's not just a problem here, but all over the country, and even in other countries. It would seem that it's not just about having a having a heavy layer of limestone that depletes. Interesting.