When Eyjafjallajökull erupted in 2010 it caused widescale travel chaos as most European air space was closed for several weeks due to ash dispersal. Following this there is now worry that Katla will erupt, as throughout historical records each eruption at Eyjafjallajökull has been followed by an eruption at Katla within a few years. Katla eruptions tend to be at least 10 times more powerful than the Eyjafjallajökull 2010 eruption. Katla usually erupts twice per century but has not erupted since 1918, further fuelling fear that Katla will erupt soon. However, little is known about what influenced the behaviour of the 1918 Katla eruption. It produced a 14 km high plume which blanketed half of Iceland in ash. What was controlling the ash production; volatiles or magma-water interaction? Where was the magma stored prior to eruption? How quickly did it rise to the surface? The eruption also produced a massive flood. The peak discharge rate, at 300,000 m3 s-1 , was greater than that of the Amazon, and was reached within just a few hours. How did the eruption manage to generate so much meltwater, so quickly? These are some of the questions, we hope to answer. To help us, we will use, amongst other resources, the lessons learnt from my PhD. This investigated the role of volatiles in determining the explosivity of subglacial rhyolitic volcanism at Torfajökull. By answering these questions, we can potentially shed light on how the next (imminent?) eruption of Katla will behave, how much warning we will have and when the most hazardous periods will be.