Even more delicate than the caves’ features are the cultural sensitivities that surround them. Many native Hawaiians consider lava tubes kapu, or sacred sites, because of their frequent use as ancient burial grounds. In Hawaiian tradition, bones contain a person’s mana, or spiritual energy, and aren’t to be unnecessarily disturbed.

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Keoni Alvarez, a 31-year-old activist and filmmaker who has battled developers trying to build atop burial caves, says that whenever human remains are found inside a lava tube, they render the entire cave system, start to finish, kapu. “We believe our caves are sacred and should not be desecrated,” he tells me. The problem is that no one can know whether a particular cave was used for ancient burials until it has been explored. Many native Hawaiians categorically refuse to venture into lava caves out of respect for what they might encounter inside.

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Pristine freshwater pools are rare in Hawaii’s lava tubes. They may look inviting, but explorers say divers can become disoriented in the twisting passages or trapped by blockages or rockfall and run out of air.

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But if modern Hawaiians tend to be wary of lava tubes, their ancestors clearly used them quite a bit. Many cave openings show evidence of prehistoric habitation, complete with hearths and sleeping terraces. In war, longer lava tubes were sealed and used as “refuge caves” to hide women, children, and elders. In some cases, stone walls were built across tube entrances, leaving a passage just big enough for a single person to climb through.

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A local expert estimates that one in two caves on the Big Island contains some sort of archaeological artifact. Especially on the dry, leeward side of the island, freshwater is hard to come by, and lava tubes were often the best place to find it. Deep inside Kanohina, hundreds of yards from entrances, one frequently comes across remains of kukui-nut torches and rings of rocks that once propped up gourds used to collect dripping water.