Besides these, Albert Meyer Reef, (20°53'S, 172°19'W, almost 5 km long and wide, least depth 3 metres, 326 km southwest) is not officially claimed by Niue, and the existence of Haymet Rocks, (26°S, 160°W, 1273 km ESE) is in doubt.Niue is one of the world's largest coral islands. The terrain consists of steep limestone cliffs along the coast with a central plateau rising to about 60 metres above sea level. A coral reef surrounds the island, with the only major break in the reef being in the central western coast, close to the capital, Alofi. A notable feature is the number of limestone caves found close to the coast. The island is roughly oval in shape (with a diameter of about 18 kilometres), with two large bays indenting the western coast, Alofi Bay in the centre and Avatele Bay in the south. Between these is the promontory of Halagigie Point. A small peninsula, TePā Point (Blowhole Point), is close to the settlement of Avatele in the southwest. Most of the population resides close to the west coast, around the capital, and in the northwest. Some of the soils are geochemically very unusual. They are extremely highly weathered tropical soils, with high levels of iron and aluminium oxides (oxisol) and mercury, and they contain surprisingly high levels of natural radioactivity. There is almost no uranium, but the radionucleides Th-230 and Pa-231 head the decay chains. This is the same distribution of elements as found naturally on very deep seabeds, but the geochemical evidence suggests that the origin is extreme weathering of coral and brief sea submergence 120,000 years ago. Endothermal upwelling, by which mild natural volcanic heat draws deep seawater up through the porous coral, may also contribute.8 - No adverse health effects from the radioactivity or other trace elements have been demonstrated and calculations show that level of radioactivity would probably be much too low to be detected in the population. These unusual soils are very rich in phosphate, but it is not accessible to plants, being in the very insoluble form of iron phosphate, or crandallite. It is thought that rather similar radioactive soils may exist on Lifou and Mare near New Caledonia, and Rennell in the Solomon Islands, but no other locations are known. The time difference between Niue and mainland New Zealand is 23 hours during the Southern Hemisphere winter and 24 hours when the mainland uses Daylight Saving Time.