Days ago, the rumbles started. They ripped through central Iceland on Saturday, home to the Bardarbunga, the largest volcanic system in the country. There were hundreds of the quakes, if not thousands. But the biggest came early Monday morning — the largest in the region since 1996.

On Tuesday, something ominous was definitely brewing in Bardarbunga. “The intense seismic activity that started on August 16 at Bardarbunga persists,” the Icelandic Meteorological Office said in a statement, adding there were “very strong indications of ongoing magma movement”.

The erupting volcano under Iceland’s Vatnajokull glacier has melted big gaps in the ice, as seen in this photo from 1996. Credit:AP

Authorities closed roads near the volcano, and the Icelandic Meteorological Office raised the risk posed to the aviation industry to orange - the second highest level. In 2010, Iceland’s Eyjafjallajokull volcano blew. Ash disrupted European air travel, ruining the travel plans of 10 million people and costing $US1.7 billion, Reuters reported.

And now it’s not a question of whether this one will blow, scientists told the Associated Press, but how. It could either blow outside the glacier, and cause damage locally. Or it could blow inside the glacier — yes, such a thing is possible — sending cascades of smoke and ash into the air that will make flying difficult for some European and American travellers.