“How do you know?”

“These are like the drip-formed limestone caverns on Earth. It takes a long time to form them.” Rachel looked around; no sign of Fraq in the dim grotto.

Zuminski said, “Earthside ones came from the shells of dead sea creatures, like the white cliffs of Dover.”

“Shells in a Martian ocean? That—”

“Right, can’t be. But limestone, it can form directly from seawater, too. In shallow areas, where there’s too much calcium for the briny water to hold. So it precipitates out in layers. Then plate tectonics scrunch it, bury it, and it forms a stone layer. Later water can creep through it from above, carrying it down in solution. Then it deposits again in stalactites like these.”

Rachel realized that of course this guy had all the background she didn’t. But…

“Mars doesn’t have plate tectonics.”

“It did,” the biologist shot back, “when it had oceans. That lasted maybe a billion years, tops. This cavern must have formed when the oceans froze, then got covered in dust. Some water was warm enough to drip through the limestone, and hollow out this cavern.”

Rachel said, feeling like a schoolgirl, “Ah. So that’s why it looks like Carlsbad Caverns.”

She walked over to some glistening snottites, hanging from the ceiling like small stalactites—beauty with the consistency of snot.

Zuminski grinned and said, “Marsmat must remember that. Memorial! Know this place is really ancient. From before the time that the Marsmat itself evolved to intelligence. Billions of years! So it’s a holy place to the Mat? Maybe. Or maybe just a place where it deposits truly old things. Is dump, maybe. Like this—and what is it?” He pointed at the strange metallic object, standing at the center of an exact circle of pale-ivory glowing mat.

Rachel walked carefully to it, crouched down, turned off her headlamp. Three round bumps near the top caught her eye—shiny, reflecting. “These look like crystals—seems to be a weak glow behind them.”

“Some energy source running it,” Zuminski said.

“The Mat didn’t make this, but Fraq must have gone searching for it.”

“How come we didn’t find this?” Zuminski’s mouth twisted ruefully. “Might be, never explored this way. Doubtful, though.”

“Or maybe…” She pointed to the precise circle formed by the Mat. “Could the Mat have covered it, when a human team went through?”

Zuminski touched the lip of the Mat with a finger. It jerked back, a surprisingly quick reaction. “Hey!—sensitive. Fast. So… now… maybe slid off this object—”

“When Fraq gave it a signal.” Rachel shrugged. “Somehow.”

To Rachel the thing looked old, even in this setting. She had imagined there might be some quarry Fraq was seeking, but—this? Its very existence implied another thread of speculation, and she made a swift decision.

Rachel stood up, taking quick photos with her shoulder-mounted camera, and said, “Let’s find Fraq. Maybe he’ll ’fess up.”

“May be more…” Zuminski went off down the grotto, headbeam stabbing into the thick, murky fog. He did glance back at her for a long moment, as if suspecting something, before he continued on his way.