A house on Hanoi’s outskirts has sunk more than four meters and officials said an underground karst system might be the cause.

The two-storey house in An Tien Commune, My Duc District, 1.5 hours south of downtown Hanoi, sunk an additional 20 centimeters during the weekend after it had subsided almost four meters to its back early this month.

The collapse happened after a pit of 30 square meters (320 square feet) wide appeared out of nowhere behind the house. The 110-square-meter house was built in 2017 at a cost of over VND800 million ($34,400).

Three other houses nearby have developed cracks in their walls and yards.

No injuries or deaths have been reported and the authorities have evacuated residents in the four houses to safety.

A house on the outskirts of Hanoi has sunk over four meters and leaned to its back.
Photo by VnExpress/Tat Dinh.

Tran Van Hoanh, chairman of An Tien Commune, said the subsidence had happened near the area where another collapse had happened in April 2016.

Experts from the municipal Department of Natural Resources and Environment had said then that the area was home to an underground karst system. The National Geographic defines karst as a landscape shaped by the erosion of limestone.

The system in Hanoi combines around 11 caves and many streams. The subsidence happened because the streams had run out of water, experts suspect.

"For the latest accident, it needs investigation from the department but it is possible that the system is still the reason," Hoanh said.

Following the commune’s report, the department has sent a team to investigate the incident.

The commune will wait for the department’s findings to see if more households have to be evacuated from the area.