The appeal of caves is, obviously, primal. They offer, in their darkness, both an instant physical reward — shelter — and something more metaphysical. For as many millenniums as there have been humans, caves seem to have been considered a contact zone with the magical, the otherworldly, the irrational, the unconscious. Prehistoric people used them as burial grounds and ritualistic art galleries. The Greeks built shrines and oracles in them and populated them with fictional monsters. (Odysseus’s Cyclops lived, with his flock of giant sheep, in a cave.) Ancient Buddhists dug out caves everywhere — 30 at the base of an Indian waterfall, 500 in a mountain at the edge of the Gobi desert — and stuffed them with their most elaborate art. Christ was entombed in, and then resurrected from, a cave. The Dead Sea Scrolls were found buried in 11 different caves. It’s no accident that walking into a great cathedral or mosque feels like entering a giant aboveground cavern.

But it doesn’t take religion to sanctify a cave. In fact, caves challenge any common-sensical division between secular and sacred. A cave is a paradox: a place defined by its absence. It operates on a time scale that we can’t even begin to comprehend — a time scale that is, in fact, obscene to any species that cares about life and tends to measure things in minutes and years and decades. The formation of a cave is appallingly incremental. Most often it happens when water, trickling down through the air and the ground, picks up carbon dioxide, creating a very weak acid. This acid finds its way into the tiniest of cracks in the rock and begins, very weakly, to dissolve it. After a million years or so, this nibbling forms a nice-size cave. Stalactites and stalagmites, created by minuscule mineral deposits left by single drops of water, form at a rate of roughly one cubic inch per 100 years.

The tallest known stalagmite is 220 feet high.

A cave, in other words, is time showing off. Most geological features form slowly, of course, but caves seem extramiraculous because of the intricacy, the beauty and the delicacy of the structures — all created not by plate tectonics or giant rivers but by individual drops of water. It’s like painting the Sistine Chapel with an eyelash.

Today, in the omnipresent data storm of the 21st century, the primal appeal of caves takes on a new dimension. The earth, including the ocean floor, is now comprehensively mapped. Caves are not. Google’s camera cars have yet to drive inside them. They remain blank spaces. In a world of instant access, caves are the very definition of slow. In a world of constant presence, caves are aggressively absent. In a world of superficiality, they are profound — literally profound, in the original sense of “deep.” (Latin profundus: “before the bottom.”) This means that we’re even more drawn to them because they preserve something precious that’s becoming hard to find: ignorance, blankness, the integrity of total silence. Today, given that we can know just about anything, a cave is even more of a cave.

My parents divorced when I was 4, and one of my dad’s favorite activities, when he had us on weekends or during school vacations, was to take us to caves. He took us to Mercer Caverns, a three-million-year-old hole in the ground that was once a sacred burial site (prehistoric people apparently used to roll dead bodies in) but was turned into a tourist destination by a gold-hunting miner in the 1880s. He took us to Moaning Cavern, which, we were told, could swallow the Statue of Liberty. We went to Oregon Caves, where I have a vivid memory of a bat flying at my face. We saw not only run-of-the-mill stalactites and stalagmites but also “angel wings” and “soda straws” and “chocolate waterfalls” and crystals that looked like sea anemones or carrots or pipe organs. We explored chambers called the Meatmarket and Godzilla’s Nostril.

Recently, probably in an attempt to recapture the lost prehistory of my childhood, I went spelunking again. A friend told me about a cave he discovered in the woods not far from where we live: a horrifying black hole that, when he saw it last winter, had a giant icicle snaggling down from the top of its mouth. He had only peeked in to the cave but had clearly been thinking about it ever since; as soon as I mentioned the subject to him, he insisted that we go explore it. We were looking, I think, for all the things people are usually looking for in caves: adventure, exhilaration, wonder, “ourselves.” (As Yoda, in “The Empire Strikes Back,” puts it when he tests Luke Skywalker by sending him alone into a cavern, the cave contains “only what you take with you.”)