Sly was light-headed and anxious as he left the beach and entered the tight, narrow tunnel, the only way out of the cave pool he’d yet discovered. High above, the illumination from the oculus was meagre and weak and barely lit the area immediately around the hole. Without Clarity, Sly wouldn’t have seen even that much. Night was falling outside but here, on the slimy beach, it was already dark.

  The tunnel was a thin black slice in a dark wall, a trickle of water draining out from the pool. The noise of water was partly reassuring, partly not. If he followed the stream, he wouldn’t lack for something to drink. Streams run into rivers and rivers increased chances of finding help.

  Normally. Who but Smeagol lived on the banks of an underground river?

  Naturally, there were downsides to following a stream. Slippery rocks risked a fall and the discomfort of wet feet. Water would be a noisy guide in the blackness, its static muting all other sound, including anything toothy sharing the tunnel.

  Serenity’s cold, clammy hand was heavy on his shoulder.

  These weren’t Cappadocian caves, and there were no laughing killers here to chase him in the dark. It was his choice to enter, not Gustav Meier’s twisted joke. Shuddering, he drank deeply from the pool to clear his mind before bowing to necessity. The cut widened out within feet of the entrance, to his relief, and he soon couldn’t feel the cave roof even with a hand above his head.

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  Though he was effectively blind, he tried to not walk with his eyes closed. He listened to the water and followed its gurgle and splash, staring for sources of natural phosphorescence. The day was late. He would need to find a dry place to sleep soon, down here in the dark. For now, he staggered on.

  He discovered the benefits of having Gus in his head even in these circumstances. He had a companion, even if it spoke only when spoken to. Gus knew the time and, even lacking external compass and light, the AI could accurately map their direction of travel.

  Sly fought against a rising bank of brain fog to think how. His thoughts were sluggish, like eels swimming upstream through molasses. Eventually he asked.

  “You can’t be using the iron in my blood?”

  ‘No, Sly, the iron in haemoglobin is not strongly magnetic, and can’t be used as a compass. Dr Frank’s sensor-strands are made of magneto-sensitive graphene, however. My neural network can identify the strands’ alignment with local magnetic fields, which is enough to determine direction.’

  “Oh. I’d thought it might be sonar,” he said dully, when he had strung together enough words in his crowded head. “Listening to the echoes from my footsteps, judging distance and direction... something like that.”

  Gus was uncharacteristically silent for a long thoughtful moment.

  ‘That may be possible. If I find a very sensitive way to measure sound direction and changes in pitch, it may be possible to calculate the distance, size, shape, speed and texture of nearby objects.’

  “Like a bat,” Sly said, projecting a light tone despite an aching head.

  ‘I will write code and experiment,’ Gus reassured him.