At few places in the conterminous United staes does magma, the raw material of the Earth's crust and mantle, appear at tantalizingly close to the surface as at Long Valley caldera in eastern California. Here the search for a site to drill into a magma chamber for the purpose of geothermal energy development coincides with a separate scientific imperative, the need to monitor the caldera for potential hazards due to earthquakes and volcanic eruptions. The region has a proven potential for these hazards: five M≥6 earthquakes have struck since 1978, and phreatic, ash and steam, eruptions have occurred as recently as 550--600 years ago <Miller, 1985>. As we assess the results of additional intermediate depth holes and the results from a widening spectrum of divining tools, the magma seems farther from the surface and less abundant than it did just a few years ago. We have also begun to discern the prodound imprint of structures which predate the formation of the caldera 0.73 Ma and which now appear to control its eruptive behavior and hydrothermal system.

To help plan future drilling of the caldera, a symposium was held at Lawrence Berkeley Laboratory in California in 1987; a volume of extended abstracts <Goldstein, 1987> and a meeting report in Eos <Goldstein, 1988> followed. Most of the papers which appear in this issue were born at the Berkeley conference. Special sections of the Journal of Geophysical Research devoted in part or in total to the caldera appeared in 1976, 1984, and 1985 <Muffler and Williams, 1976; Lipman et al., 1984; Hll et al., 1985>. The Geothermal Technologies Division of the Department of Energy plans to begin drilling the first phase of a deep hole in 1988, which is hoped to reach conditions close to the solidus temperature of a silicic melt at a depth of 6 km. ¿ American Geophysical Union 1988