Vatnajökull is the largest glacier in Iceland. Vatna is a form of vatn, in which we recognise the English word water, but it also means lake. In the plural vatn becomes vötn in nominative and accusative, vötnum in dative, and vatna in genetive (of lakes). So Vatnajökull means Glacier of Lakes. Vatnajökull is located in the south-east of the island, covering more than 8% of the country. With an area of 8,100 km², it is the largest ice cap in Europe by volume (3,100 km³) . On 7 June 2008, it became a part of the Vatnajökull National Park (marked as “Vatnajokulsthjodgardur”, its Icelandic name, on the Google map at the bottom of this post. The average thickness of the ice is 400 m, with a maximum thickness of 1,000 m. Iceland's highest peak, Hvannadalshnúkur (2,109.6 m), is located in the southern periphery of Vatnajökull, near Skaftafell National Park.
Seven volcanoes are situated underneath the Vatnajökull ice-cap and most of them are active volcanoes. Grimsvötn volcano is together with Hekla, Iceland's most active volcano since the Middle Ages. Grimsvötn last erupted in 1996, in 1998 and yet again in 2004 - there are sign that it will erupt again this year (2010). You have by now noubt realised that Grimsvötn are lakes. In fact Vtanajökull covers several sub-glacial lakes (hence the name). Grímsvötn are situated in a caldera, where only the southern rim of the 6 x 8 km caldera is exposed. The mountain is known as Grímsfjall (Grims mountain), and the volcano is also known under this name (Grímsfjall) The geothermal area in the caldera causes frequent jökulhlaups.
Jon Frimann is closely following the evolution on his blog Iceland Volcano and Earthquake Blog. Some of you may have noticed that he once referred to the situation with this heading “Glacier flood has started from Gímsvötnum”, where he indeed uses the dative (plural): vötnum.
If it comes to a jökulhlaup it will run southwards through an unpopulated area with extremely low risk of any personal damage. It may however destruct bridges and seriously damage the east-west road visble on the Google map below. An eruption will probably only emit ashes into the air, and unless abnormal weather situations should arise, it should not disturb the international air traffic.
Location of Grímsvötn = 64° 25′ 12″ N, 17° 19′ 48″ W