When the Ythri arrived, Fraq had given the Library samples of SETI messages his species had captured—a gift he solemnly delivered ceremoniously to the Great Librarian. But they were meaningless. He was just handing them some garbage prayers from long-dead religions. Which had made the Librarians suspicious, including Rachel.

Rachel shook off the memory and studied her emails.

The team back on Luna had applied their method, called “language entropy,” to the Sphinx Signal. Yes! The Sphinx was speaking a true language, and—a bonus—it had positional notations.

That made the job easier. Given that the Mat lived in a cramped universe, the Byzantine warren of tunnels and grottoes, what could it say?

“Why not a map?” Rachel muttered to herself, staring at a screen clogged with data. That woke Zuminski. She waved him back to bed, but he got up and made coffee instead, grumbling. Thinking, too, no doubt.

She read through the arcane emails, sent in scattershot excitement by the team back on Luna. No help, no map. Translating word groups, with no solid context, into a map or message was the legendary puzzle.

Yet one method worked: they’d tried translating the token groups into a timeline. That broke out into a simple, linear scale. The Mat has memory—long memory—and keeps track of time. How?

Zuminski was looking over her shoulder, reading the emails, following her mutterings. He gave her a full cup and said, “Here. Coffee makes us smarter.”

“Or think we are. How can the Mat keep track of time at all?”

He slurped at his own and said, “Way we did it. Day. Night. Stars.”

“But it’s buried kilometers down.”

“Was always? Never on surface? Even in early ocean ages?”

Click.

Rachel tried to think about life that lived long but slowly. When the Marsmat emerged onto the surface in wet eras, it might even have had some perception of the sky. It would know days and nights, maybe even the long Martian year….

Zuminski sat beside her, upright, startled. “Idea,” he said, which meant he had some Mars lore to divulge. She tried to put him off, to focus, but he went on, and slowly it dawned on her that he might be right.

Decades before, he said, in another chasm thousands of kilometers away, an earlier expedition had found odd membranes that the local Mat used. Transparent capsules filled with water, shaped just like what they in fact proved to be—lenses. Apparently the Mat was using these to examine tiny details in its own structure. That discovery had marked the first known tool use by the Mat.

“So I just thought—”

“Those lenses could also make a telescope,” she blurted.

“Could see planets, stars. Could have watched Earth.”

“And—its rich biosphere, contrasting with the glimmer of the sterile moon…” Could the Mat know envy?

“Suppose the Mat been watching Earth for entire history? Over billion years, change is slow. Continents drifted.”

She thought. Ice cover waxed and waned, sometimes nearly masking the world in a brilliant white shroud. Plants crawled onto the continents in green wealth. But then, in just a tiny sliver of the Earth’s history—the last one-millionth—the patterns of vegetation altered violently. Agriculture.