Magma represents the most extreme environment in Earth's crust. Typical conditions at the top of a magma body are > 50 MPa and > 800°C. The location and conditions of magma storage were entirely speculative until recently when geothermal drilling encountered magma in Iceland, Kenya, and Hawaii. The door is now open, if appropriate sensors can be developed, to monitor conditions at the source of eruptions. This will move eruption forecasting, a concern for 10% of the world's population, from recognizing patterns in proxy surface measurements towards the reliability of weather forecasting. It can also aid in achieving a huge increase in productivity of geothermal energy, a clean, small-footprint, bed-load source. Aqueous fluids associated with magma are superheated to supercritical, providing 10× more effective transport of thermal energy and 3.5× more efficient conversion to electricity than conventional geothermal. As an added benefit, extraction from magma of 1 GWt for 30 years would render > 1 km3 of magma uneruptable.