Exercise in the Skies
From the Super Mario Wiki
Exercise in the Skies is the first Secret Level in Yoshi's Island: Super Mario Advance 3.
The level starts out with Yoshi in a small grassy area. He soon comes to a cliff, where he has to ride various Flatbed Ferries (many of which fall off the tracks shortly after being landed on, forcing Yoshi to jump around a lot), to reach the top of some clouds, where there is a Middle Ring and a door.
Through the door is just more clouds, and after crossing a long gap, Yoshi finds another door which leads to even more clouds, as well as some Flatbed Ferries. After taking some of these and passing through the next Middle Ring, Yoshi rides the remaining Flatbed Ferries to get to a ! Switch. Hitting the switch makes red blocks appear, and jumping along those brings Yoshi to the end of the level.
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