Indonesian authorities raised the alert for a rumbling volcano on Bali to the highest level on Monday, stranding tens of thousands of travelers as ash clouds forced the closure of the tourist island’s international airport.

Mount Agung has been hurling clouds of white and dark gray ash about 3,000 meters (9,800 feet) into the atmosphere since the weekend. Video released by the national disaster agency showed a mudflow of volcanic debris and water known as a lahar moving down the volcano’s slopes.

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