In the real game, you don't. Like CoasterBGW said, you lower all the land and then build it down there, but if you want a more "underground" effect you can have a path go all the way around it right next to the tower so it shoots or drops right through it. Makes a cool effect and a great excitement rating.
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Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html