A sinkhole is caused when acidic rain seeps into the soil, making its way deep down to the bedrock that holds up the sand and soil under our feet. The process takes years, and sometimes even millennia, but eventually, the sandstone, limestone, chalk or whatever gives way and the sand or clay or dirt rushes down to fill the now vacant hole. When that happens, whatever was built on top of the sinkhole gets washed away with the sand and dirt. A sinkhole can open up at any moment, without warning. Here’s an unsettling thought: you could be sitting directly over a sinkhole in the making right now.