164 Martian years since the first colonists landed on the Red Planet, the planet sports a decently thick (500 millibar) carbon dioxide atmosphere. This, combined with a cocktail of more exotic greenhouse gases, has made the planet warm enough that the equatorial region is above freezing for half of the year. A tenuous water cycle exists, with semi-permanent lakes and streams forming in canyons and craters across the equatorial band. Snowfall is common. Shrubs, mosses, lichen, and algae have begun to colonize the warmer, wetter places.
It's spring, and the winter melt has begun. The residents of Valles Marineris don't notice. 50 M-years ago, the largest canyon system in the solar system was been tented over and made liveable. The thick tent, kept aloft by the higher pressure inside, enclosing an area twice the size of California, is truly an engineering marvel. It blocks some UV radiation and acts as a greenhouse, warming the previously firgid valley to t-shirt weather. Inside, people go about their daily lives, able to breathe Earthlike air under a Martian sky. Over 20 million people, or 40% of all Martians, call Valles Marineris home, making it the economic, cultural, and financial center of Mars.
Parts of the Valles Marineris canyon system have been allowed to remain "pristine", in order to facilitate Aerological exploration. The ever increasing snowfall has, however, begun to threaten these ancient environments. Some anti-terraforming activists have begun to call a halt to terraforming, or even de-terraforming. Valles Marineris, they say, shows that it's possible to live a comfortable, contained life on an alien world without destroying billions of years of scientific data and beauty. Advocates of terraforming say that the Valles Marineris habitat, while incredible, is far from a truly natural environment, and that as long as Mars remains uninhabitable, the lives of all people on Mars are under threat. A failure in the tent could kill 20 million people.
The Mariners are split on these issues, but have nevertheless begun to help tent over other canyon systems, basins, and craters to serve as more comfortable living spaces for a generation of new Martians. Some joke that if the Mariners have their way, the whole planet will be tented in 100 M-Years' time. The Mariners seem fine with that.