Flying under the eruption can see volcanic ash which is hanging in the sky forming these unique clouds. The eruption began on May 21, 2011 spewing tons of ash; initially the plume was over 20 kilometers high. Volcanic ash is an extreme danger to jet engines closing airspace in Iceland and parts of Europe. The photographer flew under the dark ash cloud. Grimsvotn is a sub-glacial basalt volcano and Iceland's most frequently erupting volcano. The prior eruption was in 2004.