Christmas Island

Much of the exposed surface of Christmas Island is of limestone that was formed from the million of years of coral depositing over the volcanic basalt substrata of the island. As limestone is not particularly hard as compared with the underlying layer of basalt, centuries of erosion from ocean waves, monsoon rains, cyclones and water runoffs have carved numerous fractures, caverns and sinkholes all over the island. There are more than 50 known caves found throughout Christmas Island, some of them only recently discovered, and access to many of these caves may be seasonal, intertidal or even submarine. One would expect that there would be many more caverns yet to be discovered, especially those that are submarine.