The instrument can measure the pull of gravity with such precision that a vertical movement of just a centimetre, or the state of the tides, requires corrections. Over the years the measurements reveal the breathing of the volcano as molten magma rises and falls deep beneath the surface.
Iceland isn't really land at all but rather a section of the mid-Atlantic ocean ridge that happens to be higher than the rest and which rises above the waves due to an up welling plume of hot material from the Earth's mantle. Iceland is only here because of volcanoes, and because volcanoes are constantly replenishing and extending the island. But exactly when and how a particular volcano will erupt is very hard to predict.
Askja is the biggest volcano in Iceland's north eastern volcanic region. Its last eruption, in 1961, was relatively mild, producing the long black flows of basalt that we struggled to cross. They make a jagged, fractured mass, revealing close up the flow marks you might expect from squeezing out a giant tube of viscous black toothpaste.
In 1875 things were very different. Then, rising fresh magma broke into the existing magma chamber about five kilometres beneath the volcano and "fizzed it up", releasing a huge volume of gas - like dropping sugar cubes into cola. The resulting explosion blasted out a crater 4-kilometres across, filled now by the deepest lake in Iceland. The huge quantities of light, frothy pumice or tephra it spewed into the air cloaked the surrounding landscape. The finer particles reached Europe and would certainly have disrupted flights had there been jet engines at the time. The eruption occurred in the winter, and Hazel showed me a thick layer of 1875 snow trapped beneath the insulating lava and pumice. We trod with care because the ground caves in where it melts.
So what will happen next? Preliminary analysis of the 2010 measurements suggests that a brief rise in gravity in the caldera has subsided again, but for how long? Another team we encountered, from Cambridge University, struggling with a leaking fuel can in their Land Rover trailer, has detected micro earthquakes 20 kilometres beneath the volcano, perhaps signifying magma on the move. And Askja is just one of dozens of active volcanoes in Iceland. Some, like Eyjafjallajökull and its larger but more pronounceable neighbour, Katla, lie beneath ice, posing the risk of potentially explosive eruptions as the magma reacts with melt water. Others, like Askja in 1875, can explode without external help. Further ash clouds are inevitable. The only question is when - and in which direction the wind will be blowing at the time.
Martin Redfern is a senior radio producer with the BBC World Service. He submitted this story to Science, People & Politics on 9.11.10 and it was accepted by the magazine. Copy editor: Helen Gavaghan. Editor's suggested further reading: Geodynamics, second edition, by Donald Turcotte and Gerald Schubert. Cambridge University Press 2002. I bought this book in 2003 when working on a transfer report from M.Phil. to Ph.D at The Centre for the History of Science Technology and Medicine at the University of Manchester, where I was undertaking post graduate historical research into the International Geophysical Year of 1957-1958. Martin has a degree in geology from University College, London. HG.