ever started a superflat world and wanted to expiriment on easy,mediam,or hard,but slimes kept getting in your way? or wanted to just limit the max amount of mobs per area? well now you can rid (almost) all hostile mobs with ease.the minecraft limit of hostile mobs per area is 70. so just enclose a bunch of mobs in a tube and your problem is gone.hope this helps.takes aprox. 30 sec to make and can be made underground.WARNING:this does not work if you wander to far.