With the dive underway, Marty’s nerves were on edge, and he realized he was incredibly grateful that Elsa had decided to accompany him. Cave diving solo, even for an experienced diver such as himself, was a dangerous activity. A dive buddy considerably lessened the chances of something going wrong. And Elsa wasn’t any run-of-the-mill dive buddy. Curious about her credentials, he had looked her up on the internet the night before and had been more than impressed with her accomplished career as an internationally acclaimed cave biologist. He had been saddened to learn about the death of her husband on what appeared to be her final dive. There wasn’t much information available on the exact circumstances of the tragedy, only that the man became tangled in their guideline and panicked and drowned. For something like that to happen to your spouse would be heartbreaking; for it to happen right before your eyes would be devastatingly traumatic. Marty thought he now understood why Elsa was living abroad in Sri Lanka. Presumably she’d wanted to reboot a life that had gone off the rails.

She and he were not so different after all, it seemed.

Pushing the tangential thoughts aside, Marty concentrated on the dive. The water was magnificently clear, offering twice the visibility one would expect in the open ocean on a perfect day. Horizontal layers of flow lines and ledges carved into the mineral-colored igneous rock walls marked the successively shallower depths at which the lava had flowed through the tube. The floor, created when the surface of the final stream of lava solidified, was flat and covered with a layer of muck washed down through the skylight from the rainforest above.

At the far end of the pool, where the lava tube’s roof sank below the water, they entered a tunnel about the size of those in the London Underground. The high-intensity LED lights strapped to the back of their hands cut cones of white light through the blackness. In places the terrain was silky and rounded, and in others it was rough and jagged and festooned with psychedelic lavacicles. Some were broad and tapering to points resembling giant shark teeth, while others were irregularly shaped like stretched taffy. Similar formations in limestone caves took thousands of years to develop; these would have been forged in a few anarchic hours.

Marty recalled Elsa mentioning the lava tubes on the moon, and he could very easily imagine he was not on Earth but some alien, primordial celestial body, millions of kilometers from home.