think about it, you could add a pressure plate at the bottom of a deep hole and put a spring going into it. you hook up the plate to a piston that is holding back the water. it would drown the mobs that went into the hole without any danger to you or any humans.
Very nice idea, i might do this after i make a crushing machine.
Crushing machine.
P = Piston
G = gap
R = redstone
C = cobblestone
W = water
Piston's area like on walls
PGP
GPG
PGP
behind the piston's
RCR
CRC
RCR
look from top
RPCCPR
CPWWPC
CPWWPC
CPWWPC
CPWWPC
CPWWPC
CPWWPC
CPWWPC
Each piston is connected to a redstone circuit behind the cobblestone walls, all of the redstone is connected to the piston's are connected to a lever in a watch room/control room at one of the ends of the machine. When a mob fall's into the trap, the water will drag them to the machine. They cant escape unless the player opens the machine door, witch blocks the way to the pick up post. There are 2 levers in the watch room/control room. 1st One turns the machine on and off, ON makes it crush mobs that are in the machine, OFF makes mobs not to be crushed. the 2nd lever makes the machine door open and close, It is not a good idea to open if mobs are still alive in the machine or they may attack you in the pick up post, So you should open it once all mobs die in the machine. The water will carry the drops to the pick up post witch the player can use for such as TNT and bonemeal.
EDIT: Creeper on poll has 350 votes, Lol.