"Supposedly" direct routes exist from WR and Bethesda to Fort Belvior. There are several sites within sites within sites on the post, and even with TS / TS SCI cleared CAC cards you can't get into them. And as an added bonus, the contract company SRA which became CACI which became SRA again and then CACI and then who the fuck knows what they are called now have a huge presence there. Might want to look into Chantilly (ref SRA / CACI) if you are interested, it is a "quieter" version of Mclean Va.
I have heard "stories" that Belvior, Crystal City, Chantilly, Alexandria, etc… are connected to the Pentagon as the hub since it already has deep bunkers retrofitted back in the 60s and then expanded again after 9/11. Also heard "stories" about exits under the Potomac river (flight line into Dulles / Reagan) that may or may not connect to the Airports.