That waterslide that John posted a couple of days ago sure looked dangerous, but believe it or not, there's one even MORE dangerous. In fact, it was so dangerous that it was shut down by park safety.
io9 has the story of the legendary "Canonball Loop" at Action Park in Vernon Township, New Jersey:
There was however one ride that was too extreme even for Action Park. This water slide flipped the bird at physics so rudely that it was open for a single summer and then abandoned like a drainage pipe along the River Styx. We are referring to the infamous, gravity-defying Cannonball Loop.
Due to its special status as one of the most monumentally bad ideas in theme park history, the Loop has an aura of mystery surrounding it. According to the most common reports circulating around the internet, the Loop was open for one month during the summer of 1985 before being shut down by the New Jersey Carnival Amusement Ride Safety Advisory Board. Only a few brave souls rode the loop, many of them park employees who were bribed into testing it. According to one rumor, test dummies subjected to the Cannonball Loop came out missing limbs.
Rhett Allain of Wired did physics calculation on the Canonball Loop and why it's so dangerous:
With a radius of 3 meters, this gives an acceleration of 10.2 g’s. Wow. That is just crazy. If you are going any slower, you wouldn’t make it over the loop. Any faster and you might die from the massive acceleration.