The Buildable Terrain Mod allows you to build any terrain tile over any other terrain tile. Work amounts have been changed to give it a bit of balance.
Humans have always shaped the environment to their needs, from making water streams flow wherever we want to even creating islands from scratch in the midst of the sea. With the Buildable Terrain Mod your colonists have the ability to terraform their planet too.
Buildable Terrain Mod Review
The Buildable Terrain Mod is an overpowered tool that can potentially make the game easier. It is not recommended to use in regular gameplays, but if you are creating a particular scenario that requires shaping your map somehow, this mod is for you.