KukoShakaku

I was looking through my old creative maps and found this. I had intended to build it on the server somewhere and never got around to it, but I thought I'd share some screenshots at least.

The idea was to build a functional well with a hidden passageway that would be dry when the passage was open, but would refill with water source blocks when the passage was closed. Pistons and redstone should also be hidden in both open and closed states. I tried a few different designs before finding a configuration that worked. 

The redstone takes up quite a bit of space underneath

To open the passage, the 4 visible source blocks must be removed by pulsing four pistons (three from underneath and one from the side). The blocks hiding the passage are then retracted. To close the passage and refill the well, the blocks hiding the passage are replaced while blocks on the other side hiding permanent water sources are pulsed down and then back up, allowing the water to spread new sources diagonally into the center before hiding the permanent sources again. The redstone is set up such that a single switch toggles between these states, but I don't remember anymore exactly how it works..