Mangapohue Natural Bridge is a massive cave that collapsed in on itself and has now formed two arches stacked on top of each other. It truly is a beautiful spectacle to explore.
Mangapohue Natural Bridge is about 30 kilometers past the infamous Glow Worm Caves in Waitomo. Continue on Te Anga Rd and you'll run into the beginning of the trail on the right. It's only about a 10 min walk out to the cavern so no need to worry about bringing a day pack.
You walk through a tree covered gorge of limestone with the stream running below until you run straight into the cavern. The water runs through the lower arch and you'll be able to climb the stairs to the upper arch stacked on top. The water is very cold in the stream, but on a hot day will be quite refreshing to dip in and wade underneath the massive ceiling of the cavern.
This place is a little difficult to photograph, but I suggest popping on your wide angle lens and bringing a tripod for longer exposures since it is quite dark through the gorge. Enjoy!