Bale said, “Remember, we aren’t supposed to speak to them.”

Reath snorted. “I doubt if they would understand you if you did.”

Drea asked, “What are they?”

“Specialists,” Reath said. “Like everybody here. Post-humans adapted to their roles in keeping the whole functioning.”

Berra was growing anxious. Mutely she walked back and forth along the corridor she wanted to take, away from this junction.

At last Alia took pity on her. With backward glances at the workers on the wall, she led her party away.

After another hundred paces, Berra halted. They were in a stretch of corridor as bland and featureless as the rest. But Berra patted the wall with her small hand, and a door opened up like lips parting.

Fetid air gusted out into the corridor. Hot and moist, its stench was unmistakable, even filtered by the face masks. They all recoiled, save Berra.

“Lethe,” Drea said. “That’s shit!”

Berra waited patiently by the door, her eyes locked on Alia’s face.

Alia asked, “What’s through here?”

Berra said, “The way to the Listeners.”

Reath said firmly, “Come on.” He stepped forward through the door.

Alia reluctantly followed — and was immersed in a fog of stinking air. For the first time she was grateful for her face mask.

The chamber was huge, its far walls lost in a mist of humidity. But the room was dominated by a tank of some fluid, so large it was almost a lake. The water was cloudy, brownish, and warm enough for steam to come coiling up from its surface. Small waterfalls erupted from the walls, spilling more fluid into the brimming pond, and big low-gravity ripples washed with low gurgles against the walls of the tank.

A head pushed out of the water. Alia glimpsed a low brow, startled blue eyes, and a wide mouth set in a monstrous face. That mouth gaped open, so the water washed into it with its cargo of sewage, and then the mouth clamped closed. A muscular back broke the water surface, with knobbly vertebrae and short hairs folded flat. As the creature swam away, vast slow bubbles broke the surface behind it.

Seer laughed coarsely. “Rocket propulsion!”

Now Alia made out a whole school of the swimmers pushing languidly back and forth through the dense mess, chewing, farting, shitting. There were breaks in the far walls through which the swimmers passed.

Perhaps this chamber was just one of a whole network that laced through the mound.

Reath smiled at Alia. “You’re starting to see it, the purpose of the place?”

“I think so. This is a sewage treatment works, isn’t it? But they don’t use machines, but people. ”

The waste of the mound community poured into chambers like this. The swimmers chewed it up, shat and pissed it out, and chewed it down again. Their organs were specialized to filter and separate organic material from water, waste from recyclable goodies.

Reath said, “It isn’t so strange if you think about it. Human mothers have always produced milk for their babies. Animals predigest food for their young — and some even eat excrement to extract minerals. The details change, but to do things with humans rather than machines is the way of communities like this. Somewhere in this mound there must be big-lunged air recyclers, waste removers, builders and demolishers, drones for carrying and fetching — even for disposing of the dead. And after all a human sewage processor isn’t likely to break down.”

“Drones,”said Bale, with an expression of disgust.

“So,” said Seer, incredulous, “if these guys keep on paddling around this toilet bowl long enough they’ll turn it into soup?”

Alia leaned down, ducked her hand at the surface of the water, and raised it toward her lips. “Needs salt, I think.”

Drea recoiled. “Oh, you didn’t. ”

Alia grinned and showed her a clean hand.

Reath said, “It would probably have been safe. Shall we go on?”

Berra led the way through the chamber and out to another corridor. Before they had walked much further they were taken through another door.

They found another lake, but this was of a white substance like milk. Through this paddled more swimmers. They were not big-mouthed and hairy-backed like the shit-eaters of the sewage lake, but more delicate, with thin limbs and big watchful heads. They had webbed fingers and toes.

And every one of them had a swollen belly.

Drea walked forward curiously. The swimmers reacted nervously, paddling away through their lake of milk.

Reath said quickly, “Check your face masks. Any pheromones in the air will be concentrated here.”

Drea asked, “What is this place?”

“Can’t you tell?” Alia pointed.

In the middle of the lake a woman leaned back, supported by two others. She lifted her bare hips out of the milky fluid, spread her legs — and babies slid out — two, three of them. The newborns swam around confidently, eyes open. They seemed to have no umbilical cords, no placentas. One of the babies seemed to be laughing, just heartbeats old.

The attendants who had helped with the delivery had bellies as swollen as everybody else here: they were all female, and they were all pregnant. And it was no particular coincidence that this woman had given birth the moment the visitors had walked in the door, Alia thought: no doubt there were births here all the time, every second of every day. This, of course, was the very heart of the mound.

“These are the mothers,” Berra said simply.

Alia understood. This was not really a human society at all. It was a Coalescence: it was a hive.

I got a call from Shelley Magwood.

She said she had fixed a meeting with Earth Inc., the nation’s largest private geoengineering concern. The purpose would be to explore ways we could leverage their expertise in macro-projects to get our nascent hydrate stabilization scheme off the ground — “Actually into the ground,” as Shelley quipped. It was a crucial step for us.

But the meeting had to be face-to-face, Shelley said. The two of us had to go out to EI’s headquarters in the Mojave Desert.

I dreaded the thought of yet more flying. I complained, “Given that these guys aspire to rebuild the Earth, demanding a meeting face-to-face is a bit twentieth century.”

Shelley, projected virtually to Rosa’s apartment, just shrugged. “Primate politics still works. Look, we need to follow EI’s lead. These guys know how to get these big projects accepted and done, and being shy about their methods at this stage isn’t going to help.” She grinned, the lively-minded engineer, curious. “Anyhow I hear they have some spectacular stuff out there.”

“Yeah, a regular save-the-world theme park,” I groused.

“Oh, come on. It’s an adventure. Anyhow they have a point. Did you know that you can’t fool a chimp with a VR? They just wave their hands through the images. They are too dumb to be taken in.”

“Or too smart.”

She reached out, as if to ruffle my hair. I flinched, I couldn’t help it. But when it hit my flesh her VR hand just broke up into pixels, little cubes of light that scattered in the air. She laughed. “Isn’t real life better? I’ll meet you at JFK. We can fly on together from there.”

I said my good-byes to Rosa.

Of course our business was unfinished, but I had caught her attention with my ghost. Rosa was a much darker character than Shelley, much more cynical and remote, and so much older, of course. But when she focused on a problem that interested her she was bright, sharp, curious, intense, just as Shelley was. They had a lot in common, I saw — even though Shelley the rationalist engineer would have been suspicious of the arcane strangeness of Rosa’s life.

I endured the hours of the flight into JFK, where Shelley met me. We only had a couple of hours on the ground before we set off again on another immense seven-league-boots jaunt to LAX, and Shelley gently coaxed me through the airport processes. Already jet-lagged, I managed to sleep on this flight, but by the time we were vomited out at LAX I felt even worse.