This laboratory is separated by a wall. One side is a control centre that can change the temperature of the other. It is in this room that the leech zombie can be defeated. Once the persistant enemy enters the control centre half, drop a blood pack in the laboratory. While the leeches feast on the blood pack, set the machine to "high" to kill the leeches. After lowering the temperature with "low", you can now obtain Card Key Level 2 from Hersh, which will allow you to access the Underpass entrance.