Head over to *Steam folder*\steamapps\common\audiosurf\engine\textures, open up the following files using your image editor of preference (even Windows' Paint will work!):
- cliff1-1.png
- cliff1-2.png
- cliff2-1.png
- cliff2-2.png
Turn them into a 1x1 pixel image (you can do this with Windows' Paint by dragging the bottom-right square all the way to the top-left one), save changes.
Congrats! You learned how to make an invisible cliff!
If you ever want to see the cliff again, follow
these instructions.