Just before the new year in 2021, the earth in a quiet area of Croatia began to act oddly. A near-perfectly round hole sprang out of nowhere in a garden in Meenani, a hamlet 40 kilometers (24 miles) from Croatia's capital city of Zagreb, measuring 30 meters (98 feet) broad and 15 meters (49 feet) deep. More and more of these huge holes appeared in Meenani and its adjacent village Borojevii during the next few weeks. According to the Croatian Geological Institute, at least 54 totally collapsed sinkholes of various sizes had been documented by the end of January. According...