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

Authorities closed roads near the volcano, and the Icelandic Meterological Office raised the risk posed to the aviation industry to 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 $1.7 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 travelers.

This was “the largest earthquake measured in the area since the eruption in Gjalp in 1996,” national seismologist Kristin Jonsdottir told local media, referencing a 13-day-long eruption that caused some serious flooding. “There’s full reason to expect an eruption.”

She told the Icelandic paper, Visir, that magma is sloshing somewhere between three and seven kilometers below the surface.

“Presently, there are no signs of eruption,” Iceland authorities said. “But it cannot be excluded that the current activity will result in an explosive subglacial eruption, leading to an outburst flood and ash emission.”