A short walk from Ngatiarua village, Vai Tango is the most-visited of Mauke's karst caves. A steep descent through a hole in the makatea opens into a cathedral-sized chamber with stalactites and a deep, cold pool of fresh water at the bottom. Bring a torch and reef shoes — the floor is slippery — and swim across the pool to a hidden side chamber.
The cave is on private land; your guesthouse host will arrange permission and lend a guide. Skip if you are claustrophobic.