More than likely if you're reading this article, you've ridden the Tower of Terror once and you may recall in the ride sequence, your elevator moving forward out of the first drop shaft through a scene with eery Twilight Zone decorations and into the fifth dimension. So what is the fifth dimension and why did Imagineers add it?
The idea for the fifth dimension comes from a Twilight Zone episode called "Little Girl Lost". In the episode, a little girl named Tina falls into another dimension through the wall behind her bed. Her parents spend the rest of the episode trying to locate her, figuring she has fallen into the fourth dimension.
For the Tower of Terror, Imagineers had the idea to mimic the plot of that Twilight Zone episode and allow guests to pass into another dimension. As your car stops at a floor that appears to be a maintenance room, suddenly the fifth dimension opens up and you see an endless field of stars and your elevator starts moving horizontally forward into the fifth dimension.
Here, we see an odd assortment of sights and sounds that would make no sense in our dimension, but in the fifth dimension, anything is possible. When your elevator moves into the final drop shaft, you see another star field split and opens much like elevator doors and now you have moved beyond even the reaches of the fifth dimension and as Rod Serling says, "You are about to discover what lies beyond the fifth dimension, beyond the deepest, darkest corner of the imagination, in the Tower of Terror."
The fifth dimension scene is a major aspect of the ride today but when the Tower of Terror was being first developed, early plans called for the fifth dimension floor to be in the basement, having descended from the corridor scene, and then to rise to the top of the building and move into the drop shaft without a show scene. The idea for an elevator car that moved forward allowed Imagineers to increase ride capacity by creating 2 drop shafts, so there could be elevator cars dropping while others are loading.