A dynamic changing arena, fully redstone controlled. Many people would still use stacks of torches to create this effect, that's why I included a tutorial in the video. One should learn from a tutorial, so I explain how I did it, instead of holding your hand and placing every block one by one.
There are blocks on top of chains of signs, which go back to a single sand block, which is held up by a torch, which can be washed away by water, controlled by redstone. This happens both above- and below ground, trails left behind by the under ground signs, are covered up by the above ground signs. Of course this can't be reset.
The idea behind this was to make an increasingly more difficult challenge map, with waves of mobs coming to get you, and every few waves a new map is made. The changing map part is done, and part of the mob controll was done. But I got tired of it and stopped all work on it. Besides, a future update is bound to fix it. If either redstone controlled water, signs holding up gravitiy sensitive blocks or stackable signs are fixed, then this project won't work anymore.
Credit to Etho, as I leared from him that you could stack signs on eachother.