MECENCANI, Croatia—After the deadly earthquake came the sinkholes. A central Croatian region about 25 miles (40 kilometers) southwest of the capital, Zagreb, is pocked with round holes of all sizes, which appeared after December’s 6.4-magnitude quake that killed seven people and caused widespread destruction. Scientists have been flocking to Mecencani and other villages in the sparsely-inhabited region for observation and study. “These are so-called dropout sinkholes, and they appeared because of the specific geological composition of this area, as the soil rests on limestone rocks heavily saturated with groundwater,” said geologist Josip Terzic of the Croatian Geological Survey. While the appearance of sinkholes is not unusual following strong seismic activity, residents have been baffled by their number—about 100 have been spotted over the past two months—and the speed at which they emerged after the main earthquake on Dec. 29, which was followed by a series of aftershocks. Geologists have said …