There was a faint luminous quality that outlined the shore of the lake. He studied the line of demarkation, then guessed that the faint luminosity must come from microscopic plant or animal life that clung to the rock underwater. Sea water had a phosphorescence sometimes for the same reason.
His eyes followed the faint line up the shore in the direction he had been traveling. The silver phosphorescence turned a faint yellow. Almost out of the range of his vision the yellow was picked up by the water, like the dimmest moonlight.
He studied it for long minutes, trying to figure out the reason for the phenomenon, then he almost leaped out of his skin. The water was reflecting the yellow light! It didn't come from the water the way the luminous silver did!
He got to his feet. Reflection meant man-made light!
It was hard to follow the faint yellow light. When he switched on the infrared, the light vanished completely. When the infrared was off, he couldn't find his way.
He compromised, going a hundred feet or so with the infrared on, then turning it off and sitting quietly until his eyes adjusted themselves and he could see the yellow glow once more. After he did this a few times he could see that the light was growing slightly stronger.
Then, as he progressed, he realized why he couldn't see the source of the light. It was around a corner of the rock wall.
After several minutes of alternate walking and waiting he reached the corner. It dropped sharply into the water, and when he flashed the infrared down, he saw that the water was black. No shelf here to walk on. He debated for a moment. He could swim around, or he could try to find another way. There were plenty of cave openings. One of them might go through.
He had been lost once, and he didn't intend to let that happen again. He tore open the packet of emergency rations he had brought, searching for something with which to lay a trail.
Inside the waxed container were little cans of food and a packet of hard crackers. The crackers would do.