The peninsula is largely a low, flat, limestone tableland rising to c.500 ft (150 m) in the south. To the north and west the plain continues as the Campeche Bank, stretching under shallow water c.150 mi (240 km) from the low, sandy shoreline. Along the NW coast are is the ancient, buried Chicxulub crater, an impact site associated with the mass extinction in which the dinosaurs died out. The eastern coast rises in low cliffs in the north and is indented by bays and paralleled by islands and cays in the south; Cozumel is the largest island. Short ranges of hills cross the peninsula at scattered intervals. The only rivers are those flowing E and NW from Petén.