Mangaia

Mangaia (51.08 sq.km/31.66mi; up to 169m/554ft), the most southerly and the second-largest island in the archipelago, is in the shape of an irregular circle. The lower slopes of the central volcanic ridge are deeply indented by rivers. Round the central core are numerous irregularly shaped swampy depressions up to 366m/1200ft across, bounded at the lower end by limestone cliffs up to 60m/197ft high, sometimes dropping almost vertically. The cliffs are part of the 1.2-1.6km/.7-1mi wide plain of coralline limestone (makatea), much affected by karstic action, which surrounds the island's volcanic core. The water flowing down from the hills seeps away into dolines and extensive cave systems, to re-emerge on the shore or in karstic springs