The crevasse extended to the east like a narrow low hallway. Sax ducked under a low point, and shone his headlamp down the space between ice and rock; it extended for as far as he could see, in the direction of the east side of the glacier. It seemed possible that it might reach all the way to one of the many small caves on the glacier’s lateral edge, so after sharing the thought with Phyllis he set off to explore the crevasse, leaving her in position to be sure that any searchers who found the hole would also find someone at the bottom of it.

Outside the glary cone of his headlamp’s beam, the ice was an intense cobalt blue, an effect caused by the same Rayleigh scattering that blued the color of the sky. There was a fair amount of light even with his headlamp off, which suggested that the ice overhead was not very thick. Probably the same approximate thickness as the height of their fall, now that he thought of it.

Phyllis’s voice in his ear asked if he was all right.

“I’m fine,” he said. “I think this space might have been caused by the glacier running over a transverse escarpment. So it very well might run all the way out.”

But it didn’t. A hundred meters farther on, the ice on the left closed in and met the ice over the rockface to the right, and that was it: dead end.

On the way back he walked more slowly, stopping to inspect cracks in the ice, and bits of rock underfoot that had perhaps been plucked from the escarpment. In one fissure the cobalt of the ice turned blue-green, and reaching into it with a gloved finger, he pulled out a long dark green mass, frozen on the surface but soft underneath. It was a long dentritic mass of blue-green algae.

“Wow,” he said, and plucked a few frozen strands away, then shoved the rest back into their home crack. He had read that algae were burrowing down into the rock and ice of the planet, and bacteria were going even deeper; but actually to find some buried down here, so far from the sun, was enough to make one marvel. He turned off his headlamp again, and the luminous cobalt blue of the glacial light glowed around him, dim and rich. So dark, so cold, how did any living thing do it?

“Stephen?”

“I’m coming. Look,” he said to Phyllis when he returned to her side, “it’s blue-green algae, all the way down here.”