"Grimsvötn: 'Such a violent eruption was not expected'.
The Grímsvötn lakes (Icelandic: vötn, singular: vatn) are lakes in Iceland. They lie in the highlands of Iceland at the northwestern side of the Vatnajökull glacier and are covered by its ice cap. Beneath them is the magma chamber of a volcano. The location of the lakes is 64°25′N 17°20′W, at an elevation of 1,725 m (5,659 ft).

Grímsvötn has a southwest-northeast-trending fissure system, and the massive climate-impacting Laki fissure eruption of 1783-1784 was a part of the same fissure system. Grímsvötn was erupting at the same time as Laki during 1783, but continued to erupt until 1785. Because most of the volcano lies underneath Vatnajökull, most of its eruptions have been subglacial.

Harmonic tremor was recorded twice around Grímsvötn on 2 and 3 October 2010, potentially indicating an impending eruption.[1] At the same time, sudden inflation was measured by GPS in the volcano, indicating magma movement under the mountain. On 1 November 2010 meltwater from the Vatnajökull glacier was flowing into the lake, suggesting that an eruption of the underlying volcano might be imminent.[2] On 21 May 2011 an eruption began, with plumes 18,000 feet high and causing some 50 earthquakes." Reuters