Terraform any remaining salvageable tile.
It probably just isn't implemented yet.
Here are the most advanced terraforming improvements in the game. The first two are at the end of the branch for the Iridians, Terrans, and Drengin. The last one the Alterians get in the age of war. The in-game descriptions for these are wrong, here is what they currently do:
Biospheres: It allows you to terraform any tile, even ocean, but only one per planet.
Ultra-Terraformer: You can build multiple of these per planet and they can terraform a tile of decent quality or lower (the same level as Terraforming Plant).
Biosphere Manipulator: Same as Ultra-Terraformer, but you can only build one per planet.