In a quiet corner of Croatia just before the 2021 new year, the ground started to behave very strangely. Seemingly out of nowhere, a near-perfectly circular hole — some 30 meters (98 feet) wide and 15 meters (49 feet) deep — opened up in a garden in Mečenčani, a village 40 kilometers (24 miles) away from Croatia's capital city of Zagreb. 

Over the following weeks, more and more of these gaping holes opened in Mečenčani and its neighboring village Borojevići. By the end of January, at least 54 completely collapsed sinkholes of varying size had been reported, according to the Croatian Geological Institute. Another estimate by AFP in March suggests there were eventually as many as 100 sinkholes in and around the two villages. Some of the holes were popping up in rural fields, but many were appearing just a stones’ throw from peoples’ houses. A few were even found nestled underneath properties, causing bricks to fall and cracks to appear in buildings. 

“We feel very uncomfortable," Stojan Kresojevic, a resident of Mečenčani, told Reuters in March 2021. "The sinkholes are around us, the uncertainty is killing us. We don't know if we will be forced to leave. These holes are dangerous as they open up suddenly with water spurting out." 

Fortunately, researchers had a major clue when it comes to piecing together this geological mystery. In late December 2020, a devastating 6.4 magnitude earthquake rocked central Croatia, killing seven people and injuring 26 others. Its tremors were felt throughout Croatia, as well as in neighboring Bosnia and Serbia.

Sinkholes are not a typical symptom of seismic shake-ups, but they are known to occur in areas with hidden underground cavities and caverns — such as Croatia. Thanks to its widespread Dinaric karst geology, Croatia is home to dozens of caves, three of which are deeper than 1,000 meters (3,261 feet). These deep underground caverns are formed by draining water, which is slightly acidic, that slowly erodes the slightly soluble bedrock, such as limestone. Eventually, winding underground tunnels and caves cavities can form. A sinkhole can form among these structures if, for whatever reason, the top “ceiling” gives way and exposes the underground system.

In this recent case in Croatia, it appears the dramatic earthquake in late December literally shook up many of these precariously placed ceilings and caused them to collapse, creating dozens of sinkholes.  

“Even without earthquakes, the ground above such cavities would collapse and depressions would form, as has occasionally happened in the past, but earthquakes have accelerated and intensified these processes,” said the Croatian Geological Institute.