Oceania is the name for the huge number of islands in the Pacific Ocean north and east of the continent of Australia. The extent of the area belonging to Oceania is defined differently, for us, Australia and all islands up to Hawai'i belong to it. Hawai'i, however, is listed as a U.S.A. state.
The islands are predominantly volcanic, there are often lava tubes, but the large ones are only to be found on Hawai'i. Sea caves are always found on cliffs. But most caves are karst caves, formed in small limestone areas that are very young and formed by corals. After a period of coral growth, there is an uplift of the island, mainly volcanic, and the limestone of the reef begins to karstify.