Icelandic officials are warning airlines worldwide that one of the world’s largest volcanoes is likely in the process of erupting.

The volcano alert level of Bardarbunga has been raised to “orange” which means “heightened or escalating unrest with increased potential of eruption” according to Iceland’s volcanic scale.  The Met Office, located in the capital city of Reykjavik, says that over 250 tremors have been recorded since midnight local time.

The volcano is 15.5 miles wide and rises about 6200 feet above sea level.  The volcano last erupted in 1996, melting the glacier above it and spewing ash and molten lava.

Iceland’s last major eruption, the 2011 Grimsvotn volcanic event, caused flight cancellations across Scotland, England and Germany because of volcanic ash in the atmosphere.  The massive Eyjafjallajokull volcanic eruption in 2010 caused over 100,000 flights in Europe and across the Atlantic to be cancelled out of fear of glass particles from the eruption damaging aircraft engines.