Monday mornings are banal and pragmatic, so why not think about something mystical and mysterious for a moment, like caves? Caves are the opposite of Mondays. They're the earth's gaping maw; they eat list-checkers and task rabbits for breakfast. It's hard to take vivid photos in caves, though, because what you really want to shoot is the absence of light.

Most of us think of caves as void of light and sound, or so it seems at first to sun-minded surface beings. In truth, caves are delicate ecosystems teeming with life and natural wonders. I've seen icy stalactites, gooey stalagmites, glowworms twinkling like a night sky, gently colored waves of rocks, prehistoric wall drawings, human and animal-like formations, rushing underground streams and placid lakes in caves around the world.

The best caves require coveralls, a helmet and a headlamp to explore, but all are worth visiting. In Gardner's Gut in New Zealand, I slithered through muddy crawl spaces on my belly and worried that my shoulders would get stuck. I rappelled down slippery walls into underground rivers as deep as my knees. Artificial lighting, stairs and pontoon bridges domesticate the experience in Crystal Caves in Bermuda, but they don't diminish the experience of admiring the cave's naturally formed straws, chandeliers and human profile (Bob Marley?) one bit. Recently, in Vatnshellir Caves in Utnesvegur, Iceland, we turned off our flashlights and experienced the rich, velvety darkness that only an underground cave can offer. Some panicked and found it disorienting; others swayed a little to regain their balance. Only in the awed silence could you hear the true music of caves: the plop, plop, plop of water drips singing in the earth's inky black innards.