Here's a three-hour documentary about the making of Aliens in 1986, called Superior Firepower. Made in 2003, it features insane amounts of behind-the-scenes footage that reveal quite how ambitious a project this film was, and how much bigger it was than Alien. But that's not the only "making of" featurette on Youtube.
Here are a number of other amazing documentaries about the making of your favorite films:
The Beast Within: The Making of Alien