They use BOP's(Blow out Preventers) they keep a constant pressure inside the hole thats being drilled. They can stack several on top of each other depending on what pressure they need to maintain. The piping or drill pipe is being worked inside a tight rubber seal, the seal dose not allow any air, oil, or gas to escape into the atmospher. Made by Haliburton...
How many of you knew this? Isn't this ironic.