The high carbonate island of Niue has attracted the attention of scientists since the early European exploration of the South Pacific, but its extensive karst and numerous caves have so far received little attention. Our investigation recognizes two main types of caves on Niue: (1) steeply inclined, terraced, flank-margin caves that formed at the seaward edge of a migrating freshwater lens and that breach the vertical cliffs on the leeward side of the island, and (2) flat-roofed, water-table caves consisting of interconnected passages that developed at the paleo–water table and contain dissolution features caused by alternating vadose and phreatic conditions....