“No sound. No light. Nothing touches you except salt water, but since it’s heated to just about body temperature, you don’t feel it. It’s quiet, it’s dark, you’re weightless—and it’s all meant to help you relax and have a meditative experience.”

That’s how writer Katrina Valcourt described her first experience in a sensory deprivation tank for Honolulu Magazine—a description echoed by thousands of people who have stepped into these silent, salt-filled pods. Often called flotation tanks or isolation tanks, they’ve also earned a more evocative nickname over the decades: meditation wombs.

The likeness is easy to see.…