So the Martians had come home again, almost as small as the first time around—about two magnitudes bigger than the old ones left behind, that's right. But the relationship between the little red people and the Archaea was clearly not a simple one. Second cousins thrice removed? Something like that.
Despite this blood tie, the little red people discovered early on in their civilization that their ancestors the Archaea could be grown and harvested for food, also building material, cloth, and the like. Inventing this form of agriculture, or husbandry, or industry, allowed for a tremendous population explosion, as the little red people had just taken a step up the food chain, by exploiting the level of life just below theirs. Fine for them, and because they have helped us so much in their subtle way, fine for the humans on Mars as well; but the Archaea considered it barbarous. The little red people interpreted their sullen bovine glares as subservience only, but all the while the Archaea were looking at them thinking, You cannibals, we are going to get you someday.
And so they hatched a plot. They could see that the terraforming was just more of the aerobic same old same old; that the little red people would adapt to it, and become part of the new larger system, and move up onto the surface and take their little red place in the growing biosphere; and meanwhile the old ones would remain trapped in pitch-darkness, living off heat and water and the chemical reactions between hydrogen and carbon dioxide. It isn't fair, the Archaea said to each other. It won't do. It was our planet to begin with. We should take it back.
But how, some said. There's oxygen everywhere you go now, except down here. And they're making it worse every day.
We'll find a way, some of the others replied. We are Thermoproteus, we'll think of something. We'll infiltrate somehow. They've poisoned us; we'll poison them back. Just bide your time and keep in touch. The anaerobic revolt will have its day.
The Way the Land Spoke to Us
1. The Great Escarpment
You know that the origin of the big dichotomy between the northern lowlands and the southern cratered highlands is still a matter of dispute among areologists. It might be the result of the biggest impact of the early heavy bombardment, and the north therefore the biggest impact basin. Or it may be that tectonic forces were still roiling the early crust, and an early protocontinental craton, like Pangaea on Earth, had risen in the southern hemisphere and then hardened into place, as the smaller planet cooled faster than Earth, without any subsequent tectonic-plate breakup and drift. You would think these would be interpretations so diverse that areology would quickly devise questions that would make one or the other explanation either certain or impossible, but so far this is not the case; both explanations have attracted advocates making fully elaborated cases backing their views, and so the matter has shaped itself into one of the primary debates in areology. I myself have no opinion.
The question has ramifications for many other issues in areology, but it's worth remembering just what the big dichotomy means for people walking across the face of Mars. Hiking across Echus Chasma to its eastern cliffs gives one perhaps the most dramatic approach to the so-called Great Escarpment dividing the two.
The floor of Echus Chasma is chaos at its most chaotic, and for someone on foot, this means endless divagations and extravagances to make one's way forward. Nowadays one can follow the trail, and minimize the ups and downs, end runs, dead ends, and backtracking necessary to make one's way in any direction; and the Maze Trail is the very model of route-finding efficiency through such torn terrain; nevertheless, if one wanted to get a sense of what it was like in the early days, it is perhaps better to leave the trail, and strike out to forge a new and unrepeatable cross-country ramble through the waste.
If you do that, you will quickly find that your view of your surroundings is inadequate to plan a forward course very far. Often you can see across the land only a kilometer or less. Big blocks of chunky eroded basalt and andesite are the entirety of the landscape; it's as if one were crossing a talus whose particulates were two or three magnitudes larger than the talus one usually crushes underfoot. So that one threads through the terrain as an ant must make its way through talus. Small but unclimbable cliffs confront one everywhere one looks. The only way to make progress is to keep to ridgelines, skirting great hole after great hole, while hoping the ridgelines will connect to each other in ways that can be clambered over. It's like negotiating a hedge maze by staying on the hedge tops.
Chaotic terrain: The name is quite accurate. Here the surface of the world once lost its support, when the aquifer below it drained rapidly away, downhill and over the horizon in a great outflow flood—in this case, down Echus Chasma, round the big bend of Kasei Vallis, down Kasei's gorge canyon and out onto Chryse Planitia, some two thousand kilometers away. And when that happened the land came crashing down.