She climbed out of her car to study the prints. They’d been made by large boots-a man’s size. They led into the woods and back out again in several round trips.

She’d often heard that snow on the ground is a hunter’s best friend. She was a hunter now, following a clear trail of broken snow through the forest. She wasn’t afraid of getting lost. She had a penlight in case darkness fell, her cell phone was in her pocket, and she had the footprints to lead her back to the car. Off to the right, she heard water, and realized the streambed was nearby.

The footprints ran parallel to the stream, climbing slightly toward a massive tumble of boulders.

She halted and looked up in wonder. Melting snow had dripped down and flash-frozen again into a rippling blue sculpture of waterfalls. Standing at the base of that ancient landslide, she puzzled over the abrupt disappearance of the footprints. Had Max scaled those boulders? Wind had polished the ice to a hard glaze. It would be a treacherously difficult climb.

The sound of the stream again drew her attention. She looked down, where the running water had dissolved the snow, and saw the faint mark of a heel in the mud. If he had waded into the stream, why did his footprints not reappear on the opposite bank?

She took a step into the stream and felt icy water seep through the lacing holes into her boots. She took another step, and the water was at her boot tops and already soaking into her trouser cuffs. Only then did she see the opening in the rocks.

The cleft was partly shielded by a bush that would be lush with foliage in summer. To reach the opening, she had to wade calf-deep into the stream. She pulled herself up onto a lip of rock, then squeezed under the low entrance into the wider chamber beyond.

It was just large enough for her to raise her head. Though scarcely any light shone through the small opening behind her, she found she could make out vague details of her surroundings. She heard the steady drip of moisture and saw trickles of water glistening on the walls. Sunlight must be filtering in some other way. Was there another opening up ahead? Beyond the shadowy outline of an archway, faint light seemed to glimmer. Another chamber.

She squeezed under the arch, and almost immediately tumbled off the ledge and began to roll, down and down, until she landed hard on wet stone. Pain rang like a bell in her skull. She lay stunned for a moment, waiting for her head to clear, for the lights to stop flashing in her eyes. Something fluttered overhead and whooshed away with a beat of frantic wings. Bats.

Slowly the throbbing in her head faded to a dull ache, but the lights were still flashing in streaks of psychedelic green. Symptoms of a retinal detachment, she thought in alarm. Impending blindness.

Slowly she rose to her feet, reaching out to the cave wall to steady herself.

Instead of touching stone, her hand met something slimy and yielding. She screamed and jerked away, and more beating wings fluttered out of the cave.

It moved. The wall moved.

What she’d felt on the wall was cold, not the fur of a wriggling bat. She could still feel the wetness on her fingers. Shuddering, she started to wipe her hand on her trousers when she noticed the glow. It clung to her skin, outlining the shape of her hand in the darkness. In amazement, she looked up at the cave ceiling, and she saw a multitude of lights, like soft green stars in the night sky. Except these stars moved, swaying back and forth in gentle waves.

She stepped forward, splashing through puddles, to stand in the center of the chamber, and had to close her eyes for a moment; the swaying of those stars above her head made the ground seem to rock beneath her feet.

The source, she thought in wonderment. Max has found the source of the parasite, the cave that has probably harbored this species for millennia. Heat generated by organic decay, by the warm-blooded bodies of hundreds of bats, would keep this world constant, even as the seasons cycled on the surface above.

She took out her penlight and aimed the beam at a cluster of green stars on the wall. In that circle of light, the stars were extinguished, and what she saw in their place was a clump of worms, like a many-tentacled medusa, waving gently from the dripping stone. She turned off the light. In the restored blackness, the stars reappeared, rejoining that vast galaxy of green.

Bioluminescence. The worms used Vibrio fischeri bacteria as their source of light. Whenever this cave flooded, worm larvae and Vibrio together would be washed into the stream. Into Locust Lake. We are just the accidental hosts, she thought. A summer’s swim, an unlucky inhalation of water, and a larva would find its way through the nasal passages into a human host. There, lodged in one of the sinuses, the larva would grow, releasing a hormone as it matured and died.

That would account for the chromatographic peak in Taylor Darnell’s and Scotty Braxton’s blood: a hormone secreted by this parasite.

Tutwiler, and perhaps Anson, knew about that hormone, and about these worms, yet they didn’t tell her. They had put her and her son through hell.

In fury. she reached down, grasped a rock, and hurled it at the green stars. It bounced off the cave ceiling, clattered across the ground, and landed with a strangely metallic Clang. A fresh flurry of bats whooshed out of the chamber.

She stood immobilized for a moment, trying to process what she’d just heard.

Moving cautiously through the gloom, she stepped toward the far end of the chamber, where she’d heard the rock clang. There were not as many worms here, and in the absence of their glow, the darkness seemed to thicken and almost solidify as she progressed.

Once again she turned on her penlight and shone it at the ground. Something reflected back at her. She bent down for a closer look and saw it was a camp stove coffee cup.

Next to it was the toe of a man’s boot.

She jerked back, gasping. The beam of light zigzagged wildly as she brought it up in panic to shine on Max Tutwiler’s sightless eyes. He’d slumped to the ground with his back propped up against the cave wall. His legs were sprawled out in front of him. Froth had spilled from his mouth and dribbled onto the front of his jacket. There it joined the blacker stain of blood, which had poured from the bullet wound torn into his throat.

She stumbled backwards, turned, and splashed to her knees in the puddled water.

Run. Run.

In an instant she was back on her feet and scrambling in panic up the sloping passage to the next chamber. Bats flapped past her head. She wriggled under the archway and rolled into the entrance chamber. The sound of her own gasping echoed back at her from the walls. On hands and knees, she scuttled like a frantic insect toward the opening.

The cleft grew brighter, closer.

Then her head emerged into daylight. She took in a desperate breath of air, and looked up, just as the blow came crashing down on her skull.