The ceiling at Waitomo glitters blue-green with thousands of points of light, and every one of them is an animal hunting. The cave is limestone, in the Waikato of New Zealand's North Island, and the lights are the larvae of a fungus gnat that lives only in this country. You see them from a boat on the underground river, pulled along by hand on a rope while nobody speaks. With the last lamp out, the roof resolves into what looks like a clear night sky, doubled in black water still enough to hear the stone drip. The glow is bait. Each larva hangs up to thirty beaded silk threads below it, sticky with mucus, and lights its tail to draw insects into them. A hungry one burns brighter and dims once it has fed, and what it reels in is midges, moths and sometimes smaller glowworms. On the damp rock along the path the guide picks out single larvae, close enough for you to see the beads on the threads.
The upper levels above the river come first. Stalactites, stalagmites and rippling stone curtains stand along the lit path, each built drip by drip, and the route passes through the Cathedral, a high chamber where guides stop for the acoustics. All of that rock began underwater: about thirty million years ago it was coral, shell and seabed, heaved up into hills and then dissolved out again by rain and the Waitomo Stream. Next door at Ruakuri the same water is taken on rougher terms, and you spend three hours in a wetsuit and an inner tube, floating and jumping down underground cascades with the glowworms overhead and a hot shower at the end. What is over your head all that time has about a year to live like this. When the larva has finished glowing it becomes a small fly with no working mouth. It cannot feed, and lives three or four days on what it stored: long enough to mate, lay the next eggs and go out.