Thousands of people scrambled to flee the Congolese city of Goma on Thursday, some picking their way across landscapes scarred with lava, after authorities warned a second volcanic eruption could take place any time.

The military governor of North Kivu province, Constant Ndima Kongba, said, quoting seismic and ground deformation data, that Magma, the molten rock that normally stays beneath the earth’s crust, had been detected beneath the city and the adjoining Lake Kivu.

“Given these scientific observations, an eruption on land or under the lake cannot be ruled out at present, and it could occur with very little or no warning,” he said.

31 people perished on Saturday evening when Mount Nyiragongo, one of the world’s most active volcanoes, sent a wall of orange lava downhill towards the city, devastating 17 villages on the way.

The lava stopped just 300 metres short of Goma airport, the main hub for aid operations in eastern Democratic Republic of Congo. Much of the city was spared but hundreds of earthquakes since have opened cracks in the earth and destroyed buildings.

Ndima pointed out that 10 neighborhoods in the east of the city, which lay in the path of lava from the previous eruption in 2002, needed to move to Sake, around 13 miles (21 km) to the northwest.

“Evacuation is compulsory. Those who do not adhere swiftly carry unnecessary risks.”

Thousands of people heeded his message, filing out of town on foot, with huge bundles on their heads. Others fled by car, creating traffic jams across the city, or on large boats that took them across Lake Kivu.