This is a set of ground blocks I created back in May. These blocks enable you to transition to and from platforms and roads down to the level of the map base. All blocks replace the base, so no "3D grass" comes through.
Extract these custom blocks to Documents\Trackmania\Blocks\
Then restart your game.
In-game you'll find them under the Blocks menu (F2) in a new folder under 12-1-4-3.