I just noticed, the original picture in the Halo 3 Screenshots section of Bungie.net, the url link is "http://www.bungie.net/images/Games/Halo3/Screenshots/DLDUSEKSLKJ/Halo3_Mythic-SANDBOX_Environment-01.jpg"
Maybe "Enviroment-01" means that its one of the enviroments you can pick as a layout, adding to Heathen's theory.