The Kingdom of Tonga is a sovereign state and an archipelago in the South Pacific Ocean. It consists of 176 islands, 52 are inhabited. After James Cook visited the islands the first time in 1773 they became known as the Friendly Islands because of the friendly reception.
The islands are either low coral islands, raised atolls, or active volcanoes. While active volcanoes offer the possibility of lava caves, especially lava tubes, we could not find any trace of such caves open to visitors. But the limestone of coral reefs, especially if it was uplifted, is subject to erosion and thus contains karst caves.
A mechanism of cave formation, which requires only a minimum of elevation is sea water mixture corrosion. It starts as soon as the island is subject to rainfall and thus has a ground water body consisting of sweet water. At the shores of the island the sweet water mixes with the salt water from the sea, which causes a chemical imbalance and thus the dissolving of limestone. As a result there is a belt of caves along the coast formed by chemical processes.
The island of 'Eua has a 200 m high central spine of limestone. There is a number of spectacular pits like Ana Ahu (Smoking Cave), which has a cascade gushing into its 70 m deep entrance shaft. 'Eua has been explored by teams of British cavers.