Vatnajökull is the largest and most voluminous ice cap in Iceland, and the second largest in area in Europe after the Severny Island ice cap of Novaya Zemlya. It is in the south-east of the island, covering approximately 8% of the country. 

Under the ice cap, as under many of the glaciers of Iceland, there are several volcanoes.Eruptions from these volcanoes have led to the development of large pockets of water beneath the ice, which may burst the weakened ice and cause a jökulhlaup (glacial lake outburst flood).During the last ice age, numerous volcanic eruptions occurred under Vatnajökull, creating many subglacial eruptions.

Vatnajökull has around 30 outlet glaciers flowing from the ice cap. The Icelandic term for glacier is "jökull", and so is the term for outlet glacier. 

- Wikipedia