For terraforming in general, see Wikipedia:Terraforming.

Once the brains were awake, it was time to build a home for organic life. None of the planets in the cluster were likely globes, so the machines took a rocky ball on the edge of the Goldilocks zone and launched all the chunks of ice and carbonaceous chondrite they could find. If you think the timescale for the original mission was long, well the AIs had near-infinite patience. Piece by piece over tens of thousands of years, they built up a planet from dry stone into something that could support life.

Machines around the various stars in the cluster lensed starlight to knock matter out of stable orbits and off toward the new planet. I’ve been on expedition, and seen the lens devices with my own eyes. They’re impressive machines, like vast engines of war from our distant and more violent past. I’m told they still work, if the AIs ever choose to use them again.

The planet’s new crust slowly cooled around the freshly molten core, and this is where the Torei we know today began. The machines set down at the two poles and dug intricate tunnels through to the hot mantle beneath, boring in impossibly complicated fractal corkscrew patterns. Through coriolis forces, heat, pressure and other simple physics the magma was separated out into component elements useful for generating an atmosphere.

The machines built tetrahedral “caps” of sorts atop the vents from these tunnels, and did all of the final distillation and synthesis in them. These pyramids are apparently still there inside the current-day ziggurats. The real machinery for atmospheric generation descends down into the network of tunnels, but there is a hollow space inside the zigs where the caps can be seen.

Ah, I suppose you all have the famous image of the Dahom ziggurat in the snow. Well imagine that landscape with no snow yet, because the air was still too dry. Now imagine that the vast stepped pyramid is only one-twentieth the size, and smooth-sided. That’s how they got started. Those little pyramids worked tirelessly for centuries to generate an atmosphere suitable for garden crops and large mammals.

The ziggurats of Mazos and Dahom were built later, and we have hints that they were an act of desperation. We think that the AIs realized that they weren’t making enough progress on a self-sustaining biosphere. They created the zigs as habitat domes with the hopes that human workers could help speed up the project.