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

The Making of Predator (1987)

2001: A Space Odyssey: The Making of a Myth

From Star Wars to Jedi: The Making of a Saga (1987)

Imagining Total Recall: The Making of Total Recall

Creating The Terminator

The Making of Terminator 2: Judgement Day

Superman: The Movie

Jaws: The Inside Story