How to make a mob farm in minecraft underground

This is a simple mob grinder that you can make most anywhere.

Your materials can be anything that you wish but but these are some examples of what you can use.

Make a one block tower 24 blocks high and on the 24th block make a 3x3 hole.

You must make it at least 24 blocks high of the grinder won't work. After the hole has been made you can destroy the one block tower below the hole.

Including the block that makes the hole, the water way will be 9 blocks long and the same width as the hole.

On the outside blocks add a small wall. It should look like the third picture and to treat that everything is right so far just add three buckets of water up against the wall and if it doesn't fall through the hole everything is perfect.

Add an extra two blocks to the side of the small wall.

Add a wall that is no more and no less than three blocks high.

Make this wall all around the outside of the mob walk way.