COVER Ruins of the Serapeo market in Pozzuoli, Italy. The hard caprock of Italy's Campi Flegrei geothermal system is made up of a fiber-reinforced rock formed by the mixture of lime with regional volcanic ash known as pozzolana. The caprock shares many physical properties with Roman concrete, helping to explain the rock strains and strengths in the region. See page 617.
Photo: © Roger Ressmeyer/CORBIS