Substantia Naturae was the name given to the project and social experiment that resulted in the Twin Planets.
At its core, the project was created to study the effects that nature in our surroundings (trees, grass, bushes, etc.) have on other forms of life. Two planets were created and sent into orbit, with vastly different conditions.
The first planet, Metallokis I, was lush with plants and wildlife. It was a beautiful place, made to represent the most ideal conditions possible.
The second planet, Metallokis II, was the opposite. This planet was formed from metal, glass and stone, and any nature necessary to create fresh air to breathe was placed underground. The inhabitants were never told.