The Hellas lake

The Polar Mirrors (2460s onward). In the same decade as the Hellas Crisis, the Settlement and the Mandate together aimed mirrors at the poles of Mars, the pairing that built Pavonis. Their stated aim is thicker air and liquid water in the deep places, and they set the warming to stop short of anything that would flood the northern plains, where their own cities stand. It can do little, and slowly. Melting all the polar carbon dioxide only about doubles the air's pressure, far short of what anyone could walk in without a pressure suit; bigger gains would need comets and ice from the outer edge, which the Rock Clause allows only with the Watch's clearance. By 2497 the change can be measured but not seen: the air is slightly thicker and the dust storms worse. Every forecast agrees that the meltwater will gather first on the floor of Hellas, the deepest place on Mars: briny pools in the warm seasons from about 2550–2600, then a lasting lake under ice from about 2600–2700 (the Hellas lake), giving up the lowest districts one by one. The Open Cities call it a slow eviction. They say the rim clause of the Hellas terms, written while the mirrors were being planned, was meant to keep them in the basin where the water would go; the Settlement and the Mandate say it was only their old fear of being outflanked from the south, and that nothing floods for generations. Nobody can prove either. The Reds, a movement across the blocs, want Mars left as it is, for the record of the ancient life confirmed there in the 2190s. Hellas is about 7 km deep, so its people can climb without crossing the rim: Upper Hellas, a small Open City settlement high on the basin's inner wall, is their quiet answer to the forecast, though most of Hellas's 15 million still live on the floor.