The Mars Colonies are a group of 19 subterranean super cities built below the surface of Mars by the Brayburn Society to house all of their people, and give them a home away from Earth where they can practice science in peace. Each colony is entirely self contained, and self supporting, with each having their own localized food production, livestock, water and sewage treatment, and much more. Each colony is completely isolated from all other colonies, and the outside world, for purposes of health, disease control, and more. Each colony is also able to comfortably house and support up to one million people each, although most colonies only operate at about 80-90% capacity at any given time.
But, even though they don't need outside assistance, inter-colony trade and commerce does occur as needed, although it is somewhat limited. The colonies also act as the primary supplier of resources for [[Earthfleet[[, as well as its various support organizations, such as the Saturn Shipyards or Pluto Station. Also, while only hosting about 800k-900k people per colony, each colony itself is absolutely massive. This is because large portions of each colony is taken up with support systems, manufacturing, farmland, growing spaces, and much more. Each colony is roughly 300-400 square miles in total width and breadth, and extends down into the crush up to several miles, bringing the total space taken up by each colony to stretch into the multiple thousands of cubic miles.
But, that voluminous mass is not wasted, as many of the primary agricultural and food growing areas are themselves very large, taking up vast areas of space that mimic natural landscapes that one might find if they were to live on Earth. Areas, such as this, include vast prairie land, meadows, forests, lakes, and even rivers. Within these vast natural habitats within the colonies, large herds of mammals, birds, and so much more, roam freely within them. The reason behind the creation of these massive biodomes was the preservation of all natural animal, plant, fish and other life found natively on Earth. This was prompted by the increasing rate at which animals were going extinct on planet Earth due to over hunting, as well as expanding colonial, social, and other pressures that were being put on the animal kingdom around the late 18th century.
The first effort in this direction, began with the creation of a gene and seed back, while the Society remained in their North American colonies. DNA information, embryos, life (albeit cryogenicly frozen) specimens, and more, were put aside in a high tech sort of "Noah's Ark" snapshot of all life, from the largest, to the most microscopic, was first collected and set aside. Once the Mars colonies were established, it was determined that, for the benefit, and enjoyment of, the colonists, and to provide a testing grounds for future potential expansion out into the galaxy, massive biodomes would be created where living animals, plants, fish and much more could live, breed, and continue living safely in a protected environment, no matter what happened to their cousins on planet Earth.
The Mars Colonies, unlike some might suspect, are entirely subterranean, partially to hide them from discovery, and partially to protect them from asteroids, and other space debris that might fall on the planet, having easily penetrated the very thin atmosphere of Mars with little issue. Most colonies have between a quarter, and a half mile of surface rock that sits atop them, and separates them from the Martian surface. Since the colonies recycle everything they either produce, use or create, there are no vents at the surface through which energy, excess heat, or other items are released, as there is no need to do so. Therefore, you would not know the colonies even exist if you didn't know they were there, or even where they were located on the planet.