TOBELO, North Halmahera - At 7.40am on May 8, with the grey crater of Mount Dukono stretching before him, Mr Reza Selang started flying his drone and recording what he thought would be an ordinary video.

The volcano, on the remote island of Halmahera in eastern Indonesia, looked quiet. For days, he had been told there had been no unusual activity. His group of climbers from Singapore had spent the night near the summit after trekking up the mountain the day before.