Minecraft wiki entry for Traps, associated text: Make what is shown in the schematic, a water elevator with water only on the top block. Make sure the hole is 30-40 blocks deep, which is enough to kill the player. It could help to place a lure sign near the 'elevator' saying something like "Do not enter!" You can also make the signs' text something like a floor countdown like "35", "34", "33", etc. so players are more likely to fall for the trap. If they want, you can place hoppers with chests so you can gather the loot from the victims, but if they really want the drops, they will need a secret passage to the bottom of the pit for collection.