A red Alarm for airliners in Europe in in affect as of today. Iceland raised its aviation alert for the volcano to the highest level of red on Saturday, indicating an eruption that could cause “significant emission of ash into the atmosphere.” Red is the highest alert warning on a five-point scale.

Iceland sits on a volcanic hot spot in the Atlantic’s mid-oceanic ridge and eruptions have occurred frequently, triggered when the Earth’s plates move and when magma from deep underground pushes its way to the surface.

A 2010 eruption of the Eyjafjallajokul volcano produced an ash cloud that caused a week of international aviation chaos, with more than 100,000 flights cancelled. Aviation regulators since have reformed policies about flying through ash, so a new eruption would be unlikely to cause that much disruption.

Iceland’s Meteorological Office today says a subglacial eruption is underway at the Bardarbunga volcano, which has been rattled by thousands of earthquakes over the past week.

Vulcanologist Melissa Pfeffer said seismic data indicates that lava from the volcano is melting ice beneath the Vatnajokull glacier. She said it was not clear when, or if, the eruption would melt the ice and send steam and ash into the air.