I found the elevator and to floor B1, where the DARPA chief was sitting. The area leading up to his room was somewhat reminiscent of the Blackbird in Chrono Trigger.
We then came across MGS's "infamous" cutscenes, but...I'm generally someone who doesn't mind long cutscenes in video games at all, and Snake's conversation with the DARPA Chief definitely revealed a lot of interesting and hair-raising info (really, mass producing Metal Gears?), followed by his own death later revealed to be at the hands of Psycho Mantis. Some woman (name's Meryl Silverburgh, right?) listened in on everything, opened the chief's cell, and accused Snake of killing him! And then...
..."you're shaking. You're acting like a rookie. *points gun at woman*"
But we don't have much time for talk because it's time for a firefight! It wasn't hard at all aside from the one time where I was readying my gun and the enemies responded by throwing grenades. Whoops. And then the lady ran into the elevator and Snake had a hallucination because of Psycho Mantis. Yeah, this game's major villains definitely make their presence known! Here's hoping that Mantis has some disturbing backstory (he does IIRC, but my memory may be foggy).
I went to floor B2 and got C4 and grenades (plus a refill on every other weapon aside from stun grenades), followed by a revisit to the tank hangar to use Chief Donald Anderson's keycard to unlock new rooms and obtain Snake's cardboard box. On a side note, it's going to be pretty silly not being able to use a keycard and thermal vision at the same time, but I get the feeling that that won't be as much of an issue as it could be.
Looks like I'll have to use C4 to bomb walls and reveal a secret passage. A passage which, if I remember correctly, will lead to the first boss battle.