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Our destination was Marakoopa Cave. There, a ranger named Brooke Cohn would serve as our ambassador to the creatures from below. We met her at the ranger station and observed that she was tall and wiry, with short, curly blond hair. For someone who spent so much time underground, she was remarkably tan and attractive.

Walking toward Marakoopa, Brooke explained there were different zones of life in the cave. Subterranean species were adapted to live at different depths and some lurked right outside the entrance. The cave's entryway was blocked with a steel door to control the flow of people in and out. In front of it, Brooke pointed to one of the creatures inhabiting the cave's outermost zone. “She's all legs.”

The creature was a female spider the size of Brooke's hand. From an inch-long body, her legs stuck out three inches on each side. And she was closely guarding her egg sac, a white round ball dangling from a short silk thread in a rocky crevice. “That's a Tasmanian cave spider, an ancient species.” Her closest relatives were spiders that lived in South America, dating from the time of Gondwanaland millions of years ago, when all the southern continents were connected.

As we explored the moist folds in the wall of limestone, we realized we were looking at some sort of nursery, as there were at least three egg balls, each guarded by a giant spider. The egg balls were an inch in diameter and dotted with brown tidbits—the mother spiders decorated them with tiny pieces of rotting wood for camouflage. They would guard the egg balls for eight months before their spiderlings hatched.

The male cave spiders, Brooke explained, had significantly smaller bodies. And on their second pair of legs, the males had special hooks that they used to hold the female spider's fangs open during mating. Without the hooks, the males might be eaten before mating successfully occurred— and they often were eaten right after. During the act of mating, a female's fangs dripped with venom.

To find their way in the dark, cave invertebrates are endowed with inordinately long legs and antennae. (The male cave spider's legs can reach seven inches in length.) They can also go a long time without eating— since it's often tough to get a meal in a dark cave where nothing grows except fungus—and as a result they live a long time. Scientists believe the Tasmanian cave spider may live for decades.

Brooke unlocked the door to the cave and a stream of cold air hit us. Because the temperature in the cave is always between 48 and 52 degrees Fahrenheit, a differential builds up on hot days and high-pressure cool air rushes out the cave entrance. The day we visited it was nearly 90 degrees out, and the cave's exhalation was so strong it was hard to pull the door closed. It felt like being run over by an invisible train.

Once we were inside, Brooke flicked on a light that illuminated a cave chamber. It was enormous—the size of a ballroom—with rust brown stalactites hanging down from the rocky ceiling and flowstones creating the illusion of frozen waterfalls. The chamber's far wall was smooth, forming a half-dome-shaped amphitheater, and we heard the roiling sound of an unseen, underground stream beneath us. This chamber, Brooke said, was in the transition zone. Under natural conditions, no light penetrated, though it was slightly influenced by outside temperature and humidity.

“Ready?” she asked.

Brooke switched off the light and we were enveloped in darkness. The sound of the rushing subterranean stream and the wind howling through distant, unseen cave passages grew louder.

“Give yourselves some time,” Brooke said from the dark. “Your eyes will adjust.”

Overhead, pinpoints of burning blue light began to appear. It was as if, instead of being underground, we were watching stars appearing one by one in the night sky. Or back at the Hayden Planetarium, watching the space show.

“Glowworms,” said Alexis.

“Hundreds,” Brooke confirmed. “Thousands maybe.”

The glowworms were congregating