The Valles Caldera consists of a twelve-mile-wide collapsed volcanic crater and more than ten postcollapse volcanic domes in New Mexico's Jemez Mountains. For over a century, it was safeguarded within the 89,000-acre Baca Ranch. In the year 2000, Congress passed the Valles Caldera Preservation Act, creating the Valles Caldera Trust to purchase the ranch and create a nine-member board of trustees responsible for the protection and development of the Valles Caldera National Preserve. With special permission, qualified geologists interested in volcanic processes and hydrothermal systems have been allowed to conduct research on the preserve. One of those volcanologists, Fraser Goff, collaborated with the Valles Caldera Trust to provide an accessible scientific overview of the caldera's geologic wonders. Presented in two parts, Valles Caldera first offers a summary of significant geologic events that have taken place in the Valles Caldera area. Then Goff presents the geology, volcanology, and geothermal characteristics of the Caldera and the Jemez volcanic field. Geologic terms and names unfamiliar to all but professional geologists are defined in a summarizing glossary.