You can't build flat rides underground. There is a way, however, to move them there! You need a trainer that has a "burry ride" feature, and there you go! But, without cheating, you can't build a ride underground. That was put there for the scenario only.
Remember, the designers had more control when they made the scenarios than when we play them, so they could do stuff like that.
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Cyclops. 1900 feet of CCI goodness.