Ma'uke, Cook Island.
Motuanga or the Cave of 100 Rooms is a complex system of numerous tunnels and freshwater caverns on the Cook Islands' Ma'uke. Though it's called the Cave of 100 Rooms, no one in living memory has made it all the way to the end to verify the number. Crawl through into a compact atrium with underground pool before it opens into the network of tunnels that extends much further out - all the way under the reef and out to sea, though only a few are accessible. Swim in the final underground pool and you'll hear waves crashing on the reef above you.
The most easterly of the Cook Islands, Mauke is also home to the world's largest banyan tree, covering nearly quarter of an acre.
It's around half the size of Rarotonga in circumference with a central volcanic plateau surrounded by a ring of jagged, razor-sharp fossilised coral reaching up to 1,000 metres inland- those volcanic origins give rise to rich soil leading to its garden island reputation.