"This is good practice. Luckily we came across this bunny slope the first day. I'm sure there's hairier cliffs ahead of us."

She craned her head back. Villanueva was just a blur of light at the edge of the cliff above. She suppressed a groan, leaning on a stalagmite. And this was only day one.

* * *

Ashley rubbed her back, lowering herself slowly onto her air mattress. She could hear Michaelson mumbling into the radio several yards away, giving his final report of the day. The team had discovered signs of the previous party's passage-discarded items, boot tracks in silt, scruffs on rock-and were sticking close to their trail.

She let out a long sigh, stretching. A sharp jab in her lower back protested the motion. Their progress today seemed more like a battle. Slippery mud covered most of the walkways; sharp gypsum crystals clung to her entire body like sand on a beach, and grew more abrasive with each step; steep slopes and sharp inclines impeded their forward movement, slowing them to a crawl.

Worst of all, though, was the heat. An omnipresent wet blanket that grew heavier as the day's journey wore on. She took off her headband and twisted it, wringing out a stream of sweat. She now understood how risky dehydration was in caving. She unscrewed the top of her canteen, almost empty now. Tipping it back, she swallowed the last warm drops.

"You'll have to watch your water," Ben warned. "We can't count on finding a water hole every day." He nodded toward the small lake pooled in the back half of the cavern, half hidden by an outcropping of rock.

"I knew about this water hole," she said. "It's on the map."

"True, but this is the last cavern marked on the map. From here, it's to points unknown."

"I know. I'll be more conservative tomorrow. We should remind everyone in the morning. Especially Linda. She ran out of water at lunch and has been borrowing from my canteen."

"Yours too, huh?" Ben said with a smile. "She finished the last of mine an hour ago."

"Clever girl," Ashley said. "By the way, where is she?"

"Over at the pond… getting a drink of water."

She shook her head. "Tomorrow we'll need to be more strict with rationing."

"Oh, just leave her be. I was just joking. She's over there doing a water analysis. Besides, she's having a tough time of it."

"We all are."

Ben gestured toward the two SEALs, who were setting up the campstove a handful of yards away. Light pooled around them from their lanterns. "They barely broke a sweat."

She watched as Villanueva stripped off a khaki T-shirt and wiped his face and armpits before slipping into a green vest. With a small pop, Halloway lit the butane for the campstove. Both appeared as refreshed as if today's journey were nothing more than a Sunday walk through the park, while everyone else dragged as if just completing the Bataan death march, haggard, bone-tired. Her stomach rumbled audibly.

Ben raised an eyebrow. "I'm hungry too. But there's nothing except freeze-dried beans and franks."

"Right now, that would fit the bill."

Ben grinned. "Though a beer to wash it all down… now, that would be heaven." As he sat down on his own mattress, he suddenly swatted at his arm. "Hey, something just bit me!"

"What?"

He shined a light on his arm.

She leaned over and looked at the spot. "Looks like a mosquito."

"Bloody large skeeter. Just 'bout took a chunk out of my arm."

"Quit exaggerating."

He poked her with a finger. "Wait until you get speared. Don't come crying to me."

"That's odd," she said, scratching behind an ear. "What's a mosquito doing in Antarctica? Way down here?"

Ben's expression became serious. "Good question. You don't often find insects down here. Crickets, a few spiders, centipedes, that sort of thing-but I don't think I've ever seen a mosquito."

Ashley wondered at the significance of such a discovery. "Maybe we'd better ask our biologist."

* * *

"Thanks for sharing your water today, Khalid," said Linda. "I couldn't have made it without your help."

"Anytime," he said, breathing in the dank air. He sat on a rock, watching Linda scooping water into small glass vials. He appreciated the wide furrow of moisture down the middle of her back, pasting the cotton T-shirt to her body. The clasp of her bra was visible through the thin fabric. He bit his tongue to control his rising lust.

Smiling at him, Linda stood up and sat on the boulder beside him, shaking the vial in her hand. "That last ridge was brutal. I'm glad we're done for the day."

He could feel her body heat pulsing across the hand span of space between them. They sat in silence, Linda studying the crystal surface of the pond, Khalid studying her.

"My god!" she suddenly exclaimed, jumping to the edge of the black water. "Khalid, look over here." She crouched on her knees, waving him toward her.

He crossed to her, inhaling her scent, a hypnotic perfume in the moist air. "What is it?"

She lifted a curled shell, dripping and luminescent in the lamp's glow, that had been partially hidden by a rock in the shallows. Khalid cocked his head to the side. It looked similar to a snail's shell, but it was huge. Almost the size of a watermelon.

He asked again. "What is it?"

She rolled into a seated position, cradling the large shell in her lap. "If it's what I think it is…" She shook her head and placed a hand on his knee. "If it wasn't for your insistence that we stay a little longer, I may have missed it."

Her hand was a burning ember on his knee. He fought against pulling her into a hard embrace. A tightening in the crotch of his coveralls protested his restraint. "What's so special about an empty shell?" he asked in a strained voice.

Before she could answer, voices intruded.

"I'm telling you, the damned skeeter bit worse than a snake with broken fangs."

* * *

Ben spotted Khalid and Linda crouched by the shore of the pond. He noticed Linda slip her hand from the geologist's knee just as they rounded the rocky escarpment. Ben raised an eyebrow.

Ashley cleared her throat, announcing their presence. "Linda," she said as she approached, "Ben was just bitten by an insect that looks a lot like a mosquito. We wanted your opinion."

"Oh, sure, no problem. Did you catch one?"

"Well, kind of," he said, pointing to the smashed bug still smeared on his forearm.

She smiled, taking his forearm in her hands and rotating it into the light. "You didn't leave me much to go on." She leaned in closer. "I can't say for sure. There are hundreds of species of blood-hungry midges, flies, and mosquitoes. This could be anything." She released his arm.

"I was curious," Ashley said. "Ben told me there are seldom any biting insects in caves."

Linda scrunched up her eyebrows. "That makes sense. What would they feed on? No warm-blooded species down here." She shook her head. "They must gain sustenance in some other manner, but this individual was taking advantage of a new source for lunch." She shrugged. "These caverns just get more and more curious."

She clasped one arm around a large shell. "Look at this, for instance." She held up the shell for Ashley and Ben to examine. "Do you recognize this?"

Ashley took it from her and held it up, rotating it to view it from all angles and running a hand along its spiral loop. "Looks like a mollusk shell, but I'm unfamiliar with the species. Besides, you're the biologist."

"And you're the archaeologist. If it wasn't for my study of evolutionary biology, I wouldn't have recognized it."

"Well, what do you think it is?" Ben asked, lifting the shell into his hands, curious what all the commotion was about.

"It's the shell from an ammonite, a predatory squid," Linda said. "Species Maorites densicostatus."

"What?" Ashley snatched the shell back from Ben. She examined it again with keener interest, now holding it like it was the finest porcelain. "That's impossible. This is an actual shell. Not a fossil."

Ben stared at his empty hands. "What's the big deal? What's so bloody exciting about it anyway?"

Both women ignored him. "Are you sure?" Ashley asked. "Paleobiology was not a specialty of mine."

"Yes," Linda said. "Look here, at these striations. No modern mollusk has this conformation. And look at the chambering inside. Only one species has this unique shell. Definitely an ammonite."

Ashley leaned in closer. "But what's it doing here? Ammonites died out with the dinosaurs at the end of the Cretaceous period. This is an old shell, but I don't believe it dates back sixty-five million years."

"Let me take a look," Ben said, lifting the shell. "Many caves have preserved fossils, protected from the weather. Maybe this shell is just well preserved."

Linda nodded. "Perhaps. But before the expedition, in preparation for the trip, I read up on Antarctica's wildlife. On Seymour Island not far from here, scientists discovered many ammonite fossils. Remains that dated later than the Cretaceous extinction."

"Cretaceous extinction?" Ben asked. "What're you talking about?"

Ashley answered, "About 65 million years ago, at the end of the Cretaceous period, a great cataclysm wiped out huge numbers of species, including the dinosaurs. Some researchers theorized a massive asteroid struck the earth at that time, blowing up clouds of dust that blocked the sun and chilled the planet."

"Right," Linda added. "And the paleontologists studying Antarctica now believe that Antarctica's polar vortex may have stirred the winds enough to keep the asteroid's sky-darkening particles clear of this area, sparing this continent the great extinction."

Ben interrupted. "That's all old history. So these snails survived longer than anyone thought. So what? I mean-"