Rajendra’s hand appeared at the border of the image as he fitted the mouthpiece and switched on the air supply. Of course: all this seeping liquid would build up at the bottom of the tunnel. He glanced down once, at what looked like a dark, sulfurous pool, boiling with volcanic heat; in fact, it was probably chilly and almost odorless. Munroe had been right about one thing: you really had to be there. What’s more … the tunnel wind would be weaker at this depth than at the surface, because much of the leaking rock contributing to the total airflow was now overhead. Rajendra would have no trouble noticing the difference – but the view, alone, of gas escaping at ever greater pressure, suggested exactly the opposite.

As the camera plunged beneath the surface, the image flickered and then switched to lower resolution. Even through the turbulent, cloudy water, I could still catch occasional glimpses of the tunnel wall – or at least the wall of bubbles streaming out of the rock. It was a weird, disorienting sight – it almost looked as if the water was so acidic that it was dissolving the limestone right before my eyes … but once again, that impression would have been instantly untenable if I’d been down there in person, swimming in the stuff.

The resolution dropped again, and then the frame rate fell; the picture became a series of stills in rapid succession as the camera struggled to maintain contact. Sound came through clearly enough – though I probably wouldn’t have recognized distortion in the noise of bubbles breaking against a scuba mask. Rajendra glanced down; the view showed ten thousand pearls of oxygen streaming up through opalescent water – and nothing more distant than his knees. I thought I heard him inhaling sharply, tensing himself in preparation for touching the bottom – and then I almost sent the notepad tumbling down after him.

One still showed a startled, bright red fish staring straight into the camera. In the next image, it was gone.

I turned to the woman beside me. “Did you see – ?” She had – but she didn’t seem at all surprised. The skin tingled all over my body. How thick was the rock we were standing on? How long was the cable?

When Rajendra emerged from the underside of the island, he made a noise which might have expressed anything from exuberance to terror; with a plastic tube in his mouth, and all the other acoustic complications, all I could discern was a muffled choking sound. As he descended through the subterranean ocean, the water around him gradually became clearer. I saw a whole school of tiny, pale fish cross the lantern beam in the distance – followed by a gray manta ray at least a meter wide, mouth stretched open in a permanent, plankton-straining grin. I glanced up from the screen, shaken. This couldn’t be happening beneath my feet.