From the Super Mario Wiki, the Mario encyclopedia
To access the slide, the player has to climb the mountain until they get to the Fwoosh. Immediately behind the Fwoosh is a trail of coins leading to a secret portal to the slide. If the player loses a life on this slide and jumps back into the painting, they will return to the start of the slide, provided the life lost does not result in a Game Over.
The player will fall into a rectangular room at the top of the slide. Touching the four corners of the room will result in a 1-Up Mushroom appearing, with another 1-Up Mushroom being found against the doorway. The slide begins with a straight ramp with three dips in it, followed by a turn. After this turn is a long U-shaped bend that ends in a fork; continuing on the path leads to a drop that causes the player to lose a life, so the player needs to slide onto the wooden path to avoid it. The wooden path has two gaps in it that the player will slide over and onto a bend. After the bend is a twisted tunnel, which ends at a narrow road. Past this are two final bends ending in a large jump, leading to a room with the hole to the Power Star.
In the original game, the Power Star is in an alcove with a small fence, which makes it easy for Mario to simply perform a backflip from the ledge above to obtain the Star without having to go through the slide; in the DS version, the fence covers the entire alcove, presumably to prevent the player from performing such actions, although one can still do so using Luigi's Power Flower ability.
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