Green Pass Card
It is needed for Lara to cross to an electronic island in the middle of two large pools, so she can access the Level Exit.
Lara will find it in a very large pool surrounded with catwalk. She must make her way up to this catwalk before killing the several enemies that reside up there. The Green Pass Card lies in a glass chamber. Entering this small chamber sets of the alarm once again. Lara must use a tunnel she discovered while making her way across the catwalks to return to the control room and utilize the pass card in it's slot.
Also note that in this large chamber, you can reclaim the shotgun (after you killed the guy who had it) and the Uzis (if you have collected the Jade and Silver Dragons beforehand, as the Gold Dragon is located upon the base of a pillar in the vicinity).