You need an external software like "
Trackstudio" to make this
BLOCKMIX fx
tuto I just did here : [video]http://www.youtube.com/watch?v=mcvb3xCz45E[/video]
ITs simple ;In trackmania editor, you add 1 Block on top of it (
i always choose StadiumPillar*easy to find in the external editor if its not your last block placed.) You save it.
Then you open "trackstudio"; there's a nice option that let you change the block to an other block shape from the stadium package inside the game.
So you choose "water/block" from the list and save it in trackstudio.
Back in the trackmania editor,the old pillar block is now turn to a water block.
You may not know but water/block are the bottom of the stadium pool

when put in the air it come's transparent block (water is not colored in the game (the fabrics from the white domes does the color of the water !)
Hope it helps !!
Got nice tutos everywhere :Hawk hade made one last year:
http://www.trackmania-rpg.com/166-trackstudio-block-randomization-feature.htmlFabio too (but missing pics) :
http://www.trackmania-rpg.com/227-how-to-make-simple-lakes.htmlTRACKSTUDIO WEBPAGE