The Grímsvötn area in the Western part of Vatnajökull and the Katla area in the south-eastern part of Mýrdalsjökull in southern Iceland are of special interest because of active subglacial vulcanism. In recent times eruptions have occured periodically in Grímsvötn at intervals of about ten years, the latest taking place in 1922 and 1934. In Mýrdalsjökull, Katla has etupted twice every century since 1580, the latest eruption occurring in 1918. Simultaneously, with the first phase of the eruptions, great quantities of water, in amounts approaching 10 cu.km., are liberated from the glaciers. This phenomenon is named jökulhlaup in Iceland. In the period between eruptions the craters are covered by the glacier, and little evidence of vulcanism can be seen. Since 1934 a continuous observation of the Grimsvotn area has been carried out, and since 1943 the Katla area has been included in the programme. Below is given a much abbreviated historical summary of what observations have been made and by whom: