The three small elongated Maug Islands, the largest ~2.3 km long, represent the rim of a 2.5-km-wide caldera on a submarine edifice more than 20 km in diameter. The caldera has an average submarine depth of about 200 m and contains a central lava dome that rises to within about 20 m of the ocean surface. The truncated inner walls of the caldera on all three islands expose lava flows and pyroclastic deposits that are cut by radial dikes; bedded ash deposits overlie the outer flanks of the islands. No eruptions are known since the islands were documented by Espinosa in 1522 CE. The presence of poorly developed coral reefs and coral on the central lava dome suggests a long period of general quiescence, although it does not exclude mild eruptions (Corwin, 1971). A 2003 NOAA expedition detected possible evidence of submarine geothermal activity.