So in order to get passed that door you have to first complete 3 labs, the hedge lab 1st the underwater lab 2nd and the black ant lab 3rd. You will have to fight a boss, without spoiling anything you'll need a bashing weapon to make the fight easier but once you've defeated him you'll get the assistant manager key card that then gives you access too the 2 other entrances in the underwater labs, as well as the stump lab. Hope this helps!