A cluster of around 100 sinkholes has opened up across farmland in central Croatia, some of them just metres from homes, in the aftermath of a deadly quake that rocked the area in December.

Aftershocks from a December earthquake cause sinkholes to open up across Croatian farmland
Aftershocks from a December earthquake cause sinkholes to open up across Croatian farmland AFP / -
The sudden depressions are caused by ground water eroding the bedrock below the soil
The sudden depressions are caused by ground water eroding the bedrock below the soil AFP / -

The holes started appearing days after a 6.4 magnitude quake struck killing seven people and tearing down hundreds of buildings around the town of Petrinja on December 29.

Many of the holes have filled with emerald-coloured groundwater
Many of the holes have filled with emerald-coloured groundwater AFP / -

With the largest opening measuring 25 metres (82 feet) across, the sinkholes pose yet another danger for communities still rebuilding from the disaster.

Experts warn that a spring swell in groundwater could cause further weakening of the terrain
Experts warn that a spring swell in groundwater could cause further weakening of the terrain AFP / -

The sudden depressions are triggered when groundwater erodes the bedrock below to a point where it can no longer support the soil above.

Huge sinkholes appear in Croatia after strong earthquake
Huge sinkholes appear in Croatia after strong earthquake AFPTV / Miodrag SOVILJ

A chief engineer at Croatia's national geological survey Stjepan Terzic told AFP that while sinkholes are a normal phenomenon, the high concentration of them "in such a small location" is unusual.

The quake's aftershocks -- which are still shaking the area -- caused the normally slow formation of sinkholes to accelerate dramatically.

"Thus in a short period of time numerous holes have emerged for which, in normal conditions, decades if not longer periods would be needed."

The holes, many of which look like small round ponds as they have filled with emerald groundwater, have emerged in farm plots around the villages of Mecencani and Borojevici.