When the tunnel tightened to a narrow, elliptical restriction with a shallow roof, he slowed the scooter so he could navigate the space without knocking their heads or tanks on the rocky surroundings. The lava tube opened up substantially again on the other side, and it was there they spotted their first cave-dweller: a three-centimeter remipede lacking both eyes and pigmentation. The exotic encounters became more abundant the farther they went: an albino squat lobster; a snowball sea slug; numerous small shrimp-like crustaceans and segmented worms; a school of paeony bulleye fish, their bright red color reminiscent of the molten lava which had carved out their underwater refuge so long ago.

Eventually they arrived at a gallery that was so large their lights barely reached the distant walls. At the farthest end it split into an upper and lower passage. Marty chose the upper one for the simple reason that it was the larger of the two. At the front of a lava flow, the lava behaves much like a river delta, branching off from the original flow to create smaller distributaries, and he knew that keeping to the main channel would yield the best chance of reaching the ocean.

Thirty meters down the passage, they surfaced in an air pocket trapped above the water table. The concave chamber offered a bubble of headspace, the size of which would likely vary based on the rise and fall of the tides.

Marty plucked his respirator from his mouth and said, “Having fun yet?”

Elsa removed her respirator also. “Wouldn’t want to be anywhere else.”

He checked his pressure gauge. His stage cylinder was nearly two-thirds empty, and he suggested it was a good time to switch to their back-mounted tanks. They detached the pony bottles from their harnesses, tied them off on the guideline, and resumed the dive breathing from their main tanks.

The morphology of the tunnels continued much the same as they had before, alternating between smooth and jagged surfaces that were ornamented with crystal formations and lavacicles and the occasional lava pillar. One cathedral-like cavern featured rippled walls that looked stolen from Antoni Gaudi’s gothic Sagrada Familia. Another smaller room had a sandy island in the middle of it. This reminded Marty of a tragic tale of a diver who had perished in the Sterkfontein Caves in South Africa. The young man had left the safety of his guideline and got lost in a maze of tunnels. With his air running dangerously low, he got lucky finding a small island at the end of one tunnel—or so he thought. Rescuers found his skeleton six weeks later. Marty could only imagine the terror and loneliness of sitting in the unrelenting black for days on end, starving, understanding no one was ever going to come for you. The young man clearly realized this was his sad fate, because beside his corpse, scrawled into the sand, was a message telling his wife and mother that he loved them.