They stepped through, single file. The nearby craggy corridors yawned vacant in pale Marsmat light.

Zuminski flashed his big lamps up and down the new, rough cave they stood in, cloaked in a damp fog. Flashes of blue and green seemed to signal to them. “Plenty Mat, but did the alien come this way?”

“Dunno.” Rachel checked her helmet display. “He’s still to our right. Moving, too.”

They carefully made their way through the cramped corridor, avoiding outcroppings of thick mat. Rachel thought about how the Marsmat had separated her, Zuminski, and Xiaoling off with the rest, using its irises. Why? Had the alien sent the Mat some signal they had missed? Fraq had written the coded message they’d displayed days before, using their screen—still the best way to “talk” to the Mat, the specialists thought. What had that message actually said? Fraq had given Rachel a supposed translation from his own swirly script, but how would they know if it was really what the tangle of symbols meant? The Ythri had not given much access to their written language, and even that was in several different scripts—evidence of how old the Ythri culture was.

Rachel and Zuminski made their way through dizzying labyrinths of passages—steadily deeper and deeper, warmer, ivory wreaths of moist gases wrapping around them. She sent a ping, watching the moving display. “Ah! Fraq is right in front of us. Not far.”

The murky walls spread slowly, opening like a funnel. A shroud of gray elephant-ear parted, starting a breeze blowing in their faces. Through it—and they stood in the flaring entrance to a huge vault, shrouded in thick mists and pervaded by a creamy white glow. Rachel could see almost nothing in the shadowy depths that fell away, still going down. “What’s that?” Zuminski whispered on comm.

Something glinted in their headlamp beams. They moved forward cautiously. The Mat retreated here, forming an exactly circular arc around—

“Looks artificial,” Rachel said. It seemed to be a sculpture of an odd, lustrous metal. Silvery, but with streaks of yellow and deep blue in its depths, almost like the facets of a jewel.

Zuminski said, “Has to be manufactured.”

“Or like a crystal?—grown? Maybe this is from Fraq’s species? The Mat couldn’t make it, I’d guess.”

“No mention of such in the many reports I read. No such constructions.”

Rachel ventured, “Maybe… left here long ago?” Is this some sort of mother cavern? she suddenly thought. The place where the Mat network concentrated? First evolved? Some theorists thought there should be some such original zone, as Africa was for hominids.

They stayed a respectful distance from the thing, water splashing gently against their lower legs from a slow, warm flow. It was angular and oddly curved, about a meter on a side and sunk into the rocky floor. This grotto reminded Rachel of a family vacation when she was a girl: the Carlsbad Caverns. Stalactites had speared down from a high ceiling, dripping with creamy droplets. Stalagmites answered them, rising from the hard white floor to heights above Rachel’s head. “This is old,” she said. “A primordial cavern.”