Super Mario 64/Vanish Cap Under the Moat

In the moat. You must drain it first. Go in to the basement of the castle. Follow the circular hall, and go through the door. Follow the wall on the right until you hit a door. Go through the underwater hall, and you will find a room with two pillars. butt-slam the pillars and the water will drain. Go out the newly exposed door, go right, and you will find a hole in the ground by the waterfall. Once you enter the course, you will be given a Vanish Cap, however, it is useless on the slide. Go down the slope, ride the platforms, and there's the blue switch!
 * Location

A short course with a long hill and a bunch of rotating lifts over a bottomless pit.
 * Description

Red coins
Collect 8 red coins, four of which are on the slide, so look before you slide down.