Minecraft wiki entry for Elevators, associated text: Stand on the line between the two blocks that make the top of your elevator shaft. Dig down one block under each foot (note 1). Place a sign on a block on the two-block wide wall in front of you, it does not matter which. Dig down another block depth for both blocks. Place a sign on the same wall as the first, but on the other block, and repeat all this once more so that you have a four-block deep shaft with a zigzag of signs on one of the side walls. Looking up place two water sources besides the first and second signs that you placed. You are now standing in a safe breathing space at the bottom of a working elevator that you can use to return to the top to reload water buckets, get more signs or whatever.