Falling...

...Endlessly

This is known as a shepard tone. It was created by Roger Shepard in the 1960's, and is similar to the penrose stairs, as even though it sounds as if it is falling in tone, it also sounds as if it goes nowhere, creating an audio illusion. Some people have reported having nightmares of falling after having listened to the tone in their sleep. I won't try it, but maybe you can try turning it on before going to bed, see how long you're asleep before you wake up. :p