The Waitomo Caves are a large cave system located in the North Island's King Country known as one of the premiere spelunking destinations in New Zealand. Not that you'll need to don a headlamp and bring your climbing equipment because spots because these show caves are huge draws for the average traveler.
The most popular cave is no doubt the Glowworm Cave, a luminous species that is indigenous to New Zealand. The boat ride beneath constellations created by these creatures is a fairytale-esque venture and can be combined with tours of the following caves: Ruakuri, a stalactite and glowworm spectacle known as the longest cave tour in the country, and Aranui, which has an abundance of interesting limestone outcroppings and oddities. The cost of all three together is $91 NZD and will take about four hours when done in conjunction. If you do plan on doing all three, it may help to book ahead of time; if you want to attempt any cave singularly, tours are conducted at least every hour during the day.
Then again, if you're looking for something more than the usual tourist experience, then the Blackwater Rafting Company provides that extra thrill. Guided spelunking through more technical aspects of the caverns that also include a ride on an inner-tube through various sections of the Ruakuri Cave. It's a bit more expensive than merely a walk or boat ride, but its an exceptional way to experience a half-day with the wonders of the Waitomo Caves. Further information has been provided below for your browsing pleasure.
Note:If you'd like to see this incredible phenomenon without being compressed by the tourist crowds and having to pay, come out late at night without a guide. It's a much better way to indulge in a sense of adventure while observing some stellar natural beauty.