“I remember this,” Charles said over the suit radio. “It hasn’t changed. The sand patterns are different, of course, and there have been a few slumps… but it looks real familiar. I had a favorite fossil bed about a hundred meters from here. My father showed it to me.”

Charles portioned out my share of tools to carry, took my gloved hand, and we walked away from the tractor. I saw two deposition layers clearly outlined in a stretch of canyon wall that had not slumped: a meter of brown and gray atop several meters of pale yellow limestone, and below that, half a meter of grays and blacks.

We walked across shaved flats now, covered with sand; the oldest limestones, and beneath, the Glass Sea bottom. I drew in my breath sharply, a kind of hiccup, startled at how this realization affected me. Old Mars, back when it had been a living planet… Alive for a mere billion and a half years.

Where life arose first was still at issue; Martians claimed primacy, and Terrestrials disputed them. But Earth had been a more violent and energetic world, closer to the sun, bombarded by more destructive radiation… Mars, farther away from its youthful star, cooling more rapidly, had condensed its vapor clouds into seas a quarter of a billion years earlier.

I believed — like most loyal Martians — that this was where life had first appeared in the Solar System. My feet pressed thin flopsand five or six centimeters above the graveyard of those early living things.

“Here,” Charles said, taking us into the inky shadow of a precarious overhang. I looked up, worried by the prominence. Charles saw my expression as he stooped and brought out his pick hammer. “It’s okay,” he said. “It was here when I was a kid. Can you shine a light?”

We worked by torch. Charles pried up a slab of dense crumbling limestone. I helped lift the slab away,“ twenty or thirty kilos of rock, piling it to one side. Charles handed me the pick.

“Your turn,” he said. “Under this layer. About a centimeter down.”

I swung the pick gently, then harder, until the layer cracked and I was able to finger and brush away the fragments, clearing a space a couple of hands wide. Charles held the torch.

I peered back through two billion Martian years and saw the jewel box of the past, pressed thin as a coat of paint, opalescent against the dark strata of those siliceous oceans.

Round, cubic, pyramidal, elongated, every shape imaginable, surrounded by glorious feathery filters, long stalks terminating in slender, gnarled roots: the ancient Glass Sea creatures appeared like illustrations in an old book, glittering rainbows of diffraction as the torch moved. I specked them waving in the soup-thick seas, sieving and eating their smaller cousins.

“Sometimes they’d lift from their stalks and float free,” Charles said. I knew that, but I didn’t mind him telling me. “The biggest colonies were maybe a klick wide, clustered floats, raising purple fans out of the water to soak up sunlight…”

I reached down with my gloved hand to touch them. They had been glued firmly against their deathbed; they were tough, even across the eons.

“They’re gorgeous,” I said.

“The first examples of a Foster co-genotypic bauplan,” Charles said. “These are pretty common specimens. No speciation, all working from one genetic blueprint, making a few hundred different forms. All one creature, really. Some folks think Mars never had more than nine or ten species living at a time. Couldn’t call them species, actually — co-genotypic phyla is more like it. No surprise this kind of biology would give rise to the mother cysts.”

He took a deep breath and stood. “I’m going to make a pretty important decision here. I’m trusting you.”

I looked up from the Glass Sea , puzzled. “What?”

“I’d like to show you something, if you’re interested. A short walk, another couple of hundred meters. A billion and a half years up. Earth years. First and last.”

“Sounds mysterious,” I said. “You hiding a mother deposit here?”

He shook his head. “It’s on a secure registry, and we license it to scholars only. Father took me there. Made me swear to keep it secret.”

“Maybe we should skip it,” I said, afraid of leading Charles into violating family confidences.

“It’s okay,” he said. “Father would have approved.”

“Would have?”

“He died on the Jefferson .”

“Oh.” The interplanetary passenger ship Jefferson had suffered engine failure boosting from around the Moon five Martian years before. Seventy people had died.

Charles had made a judgment on behalf of his dead father. I could not refuse. I stood and hefted my bag of tools.

The canyon snaked south for almost a hundred meters before veering west. At the bend, we took a rest and Charles chipped idly at a sheet of hard clay. “We’ve got about an hour more,” he said. “We need fifteen minutes to get to where we’re going, and that means we can only spend about ten minutes there.”

“Should be enough,” I said, and immediately felt like kicking myself.

“I could spend a year there and it wouldn’t be enough,” Charles said.

We climbed a gentle slope forty or fifty meters and abruptly came upon a deep fissure. The fissure cut across the canyon diagonally, its edges windworn smooth with age.

“The whole flatland is fragile,” Charles said. “Quake, meteor strike… Something shook it, and it cracked. This is about six hundred million years old.”

“It’s magnificent.”

He lifted his glove and pointed to a narrow path from the canyon floor, across the near wall of the fissure. “It’s stable,” he said. “Just don’t slip on the gravel.”

I hesitated before following Charles. The ledge was irregular, uneven, no wider than half a meter. I pictured a slip, a fall, a rip or prick in my suit.

Charles looked at me over his shoulder, already well down the ledge. “Come on,” he said. “It’s not dangerous if you’re careful.”

“I’m not a rock climber,” I said. “I’m a rabbit, remember?”

“This is easy. It’s worth it, believe me.”

I chose each step with nervous deliberation, mumbling to myself below the microphone pickup. We descended into the crevice. Suddenly, I couldn’t see Charles. I couldn’t hear him on radio, either. We were out of line of sight and he was not getting through to a satcom transponder. I called his name several times, clinging to the wall, each moment closer to panic and fury.

I was looking back over my left shoulder, creeping to my right, when my hand fell into emptiness. I stopped with a low moan, trying to keep my balance on the ledge, waving for a grip, and felt a gloved hand take hold of my arm.

I turned and saw Charles right beside me. “Sorry,” he said. “I forgot we wouldn’t be able to talk through the rock. You’re fine. Just step in…”

We stood in the entrance to a cave. I hugged Charles tightly, saying nothing until my hammering heart had settled.

The cavern stabbed deep into the fissure wall, ending in black obscurity. Its ceiling rose five or six meters above our heads. The fissure’s opposite wall reflected enough afternoon sunlight into the cavern that we could see each other clearly. Charles lifted the torch and handed it to me. “It’s the last gasp,” he said.

“What?” I still hadn’t recovered my wits.

“We’ve gone from alpha to omega.”

I scowled at him for his deliberate mystery, but he wasn’t looking at me.

Gradually, I realized the cavern was not areological. The glass-smooth walls reflected the backwash of light with an oily green sheen. Gossamer, web-like filaments hard as rock stretched across the interior and flashed in my wavering torch beam. Shards of filament littered the floor like lost fairy knives. I stood in the silence, absorbing the obvious: the tunnel had once been part of something alive.

“It’s an aqueduct bridge,” Charles said. “Omega and Mother Ecos.”

This wasn’t a cavern at all, but part of a colossal pipeline, a fossil fragment of Mars’s largest and last living things. I had never heard of an aqueduct bridge surviving intact.