First, generate a new world in MineCraft. As soon as this is done Save and Quit. Then head to your minecraft directory, for me it is the following:
Now go in to your saves folder, and then into World1 (Or whatever slot you generated in). There you must scroll down and find level.dat. Delete this file. Now load up your save again and voila! It's ****ed up! If you save and quit again, and then delete level.dat again, it will come up with more crazy results.
Download one I made a few minutes ago, it is seriously epic.
Pros and Cons of Glitched Maps
+Rarity of Clay decreased
+More epic surface formations
+Big ass cliffs
+Big ass holes in the middle of the cliffs Cons:
-Screws caves up underground
-Might not look nice if you are unlicky