Various types of data are gathered and interpreted by volcanologists in light of past activity.  Most important are the changes that take place at the volcano caused by the movement of magma under the surface.  Most magmatic activity precursory to an eruption cannot be sensed directly. Instead, sensitive instruments and techniques are used to detect unrest, with different types of monitoring equipment  linked to different aspects of volcanic unrest. The three different types of instruments  available in Eruption! are described in more detail in the links below.

A project funded by the U.S. National Science Foundation Human and Social Dynamics Program