Iceland's Bárðarbunga volcano could be about to erupt, say scientists after noticing that the surface of the glacier above the volcano's caldera (cauldron-like crater at top of volcano) has begun to sink by around 65 feet since last week.
Powerful tremors measuring magnitude 5.5 shook the region near the volcano on Wednesday at 05:28 GMT.
The eruption could be large depending on the amount of magma in the volcano's chamber, writes Vox.
A volcanic eruption below the glacier could produce a violent steam explosion throwing up clouds of ash into the atmosphere.
A cloud of ash from the Eyjafjallajokull volcano in a different region of Iceland had closed much of Europe's air space for six days in 2010.
Lava continued to pour from fissures in the ground but there was no ash, reports Reuters.
Besides an ash explosion, another scenario, according to the Iceland meteorological organisation, is that the eruption could melt the ice and result in flooding of the river flowing north.
Even the ongoing lava eruption on reaching back to the glacier could lead to ash explosions and flooding.
Or, the caldera could stop sinking, seismic activity could cease and lava flows decline.
The caldera spreads over 80 square km and is covered by an ice cap that is 700 to 800 metres thick.
The caldera has been sinking since last week as magma channelled through underground passages moves away from the volcano, leaving empty space beneath.