A ﬁssure is an extended surface crack or fracture that creates a distinct separation in rock. These rifts in the earth are often associated with areas of volcanic and seismic activity, and are being studied in such places as the Inyo Crater area in Long Valley, California. Fissures can form in the sides of lava domes and serve as outlets for volcanic gasses or openings through which lava ﬂows in ﬁssure eruptions. Such eruptions, through ﬁssures rather than through a central vent, have occurred repeatedly on the ﬂanks of the Hawaiian volcanoes Mauna Loa and Kilauea. Up to seventeen million years ago, extensive basalt ﬂows from innumerable ﬁssure eruptions built up the Columbia Plateau, an area of over 200,000 square miles in Washington, Oregon, and Idaho. Fissures have recently opened within the city of Phoenix, Arizona, apparently caused by a severe drawdown of the local water table.