Something I don't understand: So there are a string of Terraforming techs that each give you the opportunity to improve ONE tile. However, when you select that tile improvement task, it will show you several available site locations. So you select ONE tile to improve, and then you cannot improve any of those previously indicated tiles. WHY NOT? By showing several site possibilities, it is showing locations that would benefit by that improvement. Sort of like how placing landfills all over the plant adds that many tile's worth of space that can now be more profitably developed. In the game, it isn't until Ultra-Terraforming that a tile improvement can be used repeatedly. Even with UT, not all site-improvable tiles that are available to earlier methods are available for UT improvement.
I'll grant that given early access to improve nearly all tiles early on would too easily Gaia-fy a planet, but still. If the tech is there and the potential sites are there, people would tend to improve nearly ALL of those sites.