BALI, Indonesia - Mount Agung on the Indonesian tourist island of Bali is threatening to erupt after more than half a century of quiet. Here's what you need to know.

IS BALI STILL SAFE FOR TOURISTS?

Seismic activity from the volcano has escalated dramatically in the past two weeks but officials say there is no reason for tourists to panic and the island famous for its surfing, beaches and elegant Hindu culture is still safe to visit.

Bali's Ngurah Rai International Airport has been operating normally since the alert status for Mount Agung was raised to the highest level on Friday but a significant eruption would force its closure and strand thousands. Nearly 5 million tourists visited Bali last year.

Videos circulating on social media that purport to show the mountain already violently erupting are hoaxes; they show previous eruptions from other volcanoes in Indonesia.

Agung is in the north of the island about 70 kilometres from the tourist hotspot of Kuta. People have been told to leave a 9-12 kilometre area around the crater where frequent tremors from the volcano are being felt.

Government vulcanologist Surono, who uses one name, said the feared eruption could be huge and potentially also close airports in East Java and Lombok, according to local media reports.

WHEN DID AGUNG LAST ERUPT?

Mount Agung's eruptions in 1963 were among the most powerful and devastating last century in Indonesia, a country with more than 120 active volcanoes.

The two major eruptions, in March and May, produced a 7.5 kilometre river of lava, deadly white-hot clouds of ash, gas and rock fragments at ground level called pyroclastic flows and lahars, an Indonesian term for an often fast-moving and fatal slurry of volcanic debris and water. Some 1,100 people were killed.

Volcanologists say the March eruption hurled ash 19 to 26 kilometres above sea level and the May explosion about 20 kilometres.

Over the past 5,000 years, Agung has erupted once a century on average and about a quarter of its eruptions have been a similar or stronger strength than 1963.

Macquarie University volcanologist Heather Handley said the eruptions in 1843 and 1963 had a Volcanic Explosivity Index of about 5 on a scale where 8 would be the strength of an ancient super volcano eruption such as Yellowstone in the U.S. or Toba in Indonesia.

HOW MANY PEOPLE HAVE EVACUATED?

Disaster officials say more than 35,000 people have fled the region around the mountain and the number is rising. It includes people who left voluntarily as well as those told to evacuate from a 9-12 kilometre zone around the crater.

Villagers are staying in temporary camps, sports centres, village halls or with friends and relatives. Some return to the exclusion zone during the day to tend to livestock or shift the animals to areas further from the volcano. Others say they are selling their cows because they don't know when they'll be able to return.

National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho has praised the welcoming response of local communities on Bali to the evacuees. A government humanitarian effort is also underway with some 14 tons of aid sent.