Indonesian authorities raised the alert for a volcano in Bali to the highest level after dark smoke was seen rising up to 11,150 feet above the mountain’s summit, the BBC reported.

The island’s airport has been shut and people within 10 km of Mount Agung have been told to evacuate. The National Board for Disaster Management said explosions were being heard 12 km from the summit, The Guardian reported.

Mount Agung had killed 1,600 people the last time it erupted in 1963. The volcano started spewing ash again in September, forcing authorities to raise the alert to the highest level and 1,40,000 people living nearby to evacuate. By the end of October, the volcanic activity reduced, people returned to their homes and the alert was lowered to the second-highest level.

On November 21, Mount Agung rumbled again, forcing 25,000 people to seek shelter, but authorities did not raise the alert level till Monday morning, after the volcano started emitting “continuous ash puffs” accompanied by “explosive eruptions” and a “weak sound of boom”, the disaster management agency said in a statement on its Facebook page. “The rays of fire are increasingly observed at night. This indicates the potential for a larger eruption is imminent.”