Lihir Island, the largest of an island group north of New Ireland, is a Pliocene-to-Holocene volcanic complex of several overlapping basaltic stratovolcanoes. The youngest, Luise, contains an elliptical 5.5-km-wide caldera that is breached by the sea on the NE side as a result of edifice collapse about 0.4 million years ago, forming Luise Harbor. The flanks are only moderately dissected, and the steep-sided caldera wall rises to 700 m above sea level. A central lava plug is strongly hydrothermally altered and displays extensive thermal activity along its margins. Thermal activity includes boiling hot springs, mud pools, and sulfur-encrusted low-temperature fumaroles. The Ladolam hydrothermal deposit hosts one of the youngest and largest gold deposits in the world, which is now being extracted by open-pit mining. The near-surface, epithermal gold deposition extends to about 400 m below sea level over an area of about 2 km2.