SimMars is a mod for SimCity 4 taking the game to Mars. It will bring new buildings, transportation systems, user-interface and music and will replace the Maxis SimCity 4 content as far as possible, giving you the feeling to play a colony on Mars.
SimMars BETA 2 brings the mod a step further to the goal. Most MAXIS content is disabled now, although not yet completely. Buildings are functional and growable. There is a first basic set of BATs for growing a city. Transportation and automata have not been transformed yet apart from street and road textures and the bus station as first mass transit. As you can see BETA 2 is not a finished product and is intended mainly for testing purpose.
You can download this mod here.