"Three sources of light," he intoned. "Light is more important than food or water. Your headlamp." He pointed with a long pale digit that looked well suited to a creature living deep underground. He waited. Apparently he wouldn't continue the lecture without classroom participation, so Anna nodded obediently.

"With spare batteries and bulbs. A flashlight." He pointed to a neat blue Maglite, brand-new and jewel-toned. "And what's your third source?" The tinted lenses winked at Anna, and she wondered if she should raise her hand before speaking.

"A candle?" she ventured, thinking of Tom Sawyer and Becky Thatcher.

That was the wrong answer Timmy had been fishing for. "Anachronism," he said triumphantly. "A candle in Lechuguilla is akin to a firefly in a whale's gullet, charming but not illuminating."

"We carried candles for years," Lisa volunteered. "When we switched out we noticed neither one of us had thought to bring matches." She laughed, a high whiffling sound. Her husband was not amused.

"Third source: another flashlight. More batteries." He stowed the lot away in the bottom of the pack.

Anna picked up a wide-mouthed plastic bottle from the pile. The top was a white screw-on cap with the letter "P" written on it with a Sharpie permanent marker. "What's this?"

"Just what it says," Timmy replied. "You pack it in, you pack it out."

"There's one urine dump near the permanent camp on the way out," Lisa added helpfully. "If you need to you can dump it there."

"Don't use it," Timmy said. "Pack it out."

From her brief exposure to caving literature, Anna half remembered discussions on how the salts and sugars of human wastewater could, over time, alter the cave environment significantly. A filtering system to remove these components from the waste so only pure water would be left behind was in the works but was yet to be realized.