The company that pioneered the first aqualoop water slide, Aquarena, have taken the concept further by doing double-looping water slides! As far as I have learned, there are at least three double-looping slides in the world; each one in Europe.
The first is the most popular for being the first double-looping slide ever built. The L2 at Wörgler Wasserwelt in Wörgl, Austria.
The second is called Pegasus II and is located at Kristall Palm Beach in Stein, Germany.
The last one, the Hurricane Loop, is located at Miramar in Weinheim, Germany.
They all look pretty intense! Would you dare drop in on any one of these slides?!