You know that missile tower ? You can jump on those greenish big metal beams. From those beams you can jump to some tiny ledge above the tunnel which leads directly under the missile. Also, not sure if it was shown in this video and I'm too lazy to check, you could also crawl into some vents at one...

Uh, the video go taken down before I had a chance to watch it.

Can someone explain this one for those that missed it?