Enter it, and follow the hallway until you get to the bloody pool. Go up the stairs, and follow the hall until you get to the Skull key door. Go back outside, and enter through the Loading entrance (there's a hall that's quicker, but this is easier to explain). Go down that passage, and open the door at the end to pick up the Skull key. You'll find a passage under a 'No Admittance' sign.
Go in through the regular entrance, and make your way forward. Enter the hole, and go down the stairs to 'Feinman Meats'.
'As you enter the second room, you'll see a crack in the wall blow it open.
