Kahmunrah's army descends onto the museum.

The most challenging element to bring to the screen involved the film's villain, Kahmunrah (Hank Azaria), an Egyptian Pharoah who plots to bring his 3,000-year- old army of the undead into the modern world. The army is glimpsed at in a place called the "Netherworld," a concept that Chen had to define visually.

"They wanted to show eternity," says Chen, "They also wanted to show these Horus guards or soldiers that transform from flapping birds to human soldiers with hawk-heads like the ancient Egyptian God. It was particularly difficult, because we're trying to put a face on something that is very abstract. They did a number of concept paintings that were very detailed and specific about what it should look like. Even getting from that concept art to the final shot was a long exploration."

Also an enormous challenge was the film's giant octopus, something that Chen believes to be one of his proudest accomplishments.

"That's one of those characters that's a particularly hard animal because, in addition to having more limbs than most, and having eight different tentacles, there's a lot of freedom of motion in that it doesn't necessarily have a skeletal structure or bones. The rig that we had to make for it is extremely complex. There's a lot of movement in the tentacles themselves, plus there's a lot of close-ups and the character itself is quite big. You have to have controls for things like the suckers and the gill and the blowhole."

To add to the challenge, the Octopus' photoreal look prevented obvious facial expressions that the director wanted to be communicated to the audience. In the end, Chen thinks that they managed to hit just the right balance of being anthropomorphized without looking or feeling overly cartoony.

Other new characters introduced for the sequel include flying Cherubs (whose faces the team modeled after the Jonas Brothers) and a bunch of talking Albert Einstein bobbleheads that Daley comes across in the gift shop of the Air and Space Museum.

Major works of art like the Thinker and Venus come to life in Smithsonian.

"[Even more] didn't make it into the film," adds Chen, "There's a whole sequence in the National Gallery of Art where a lot of the sculptures come alive. We have some of our main ones like the Thinker or Venus or Degas' Little Dancer, but there are some minor ones like a Noguchi globular thing. There's a spiral man. There are a lot of background sculptures that were only going to be used in a few scenes. Some of them got built and they got cut from the film or the director didn't like certain sculptures."

With about 535 effects shot having made the final cut, Chen found his team working hard right up until the delivery date, including a big shot that came about just two weeks from the final deadline.

"It was one of the Horus gods emerging from the netherworld. The studio felt that they didn't have a big sort of payoff. We never got a really close look at the heads of these guys so we put in a nice close-up of one of these hours guys walking toward camera and having a big squawk. It was kind of an extreme close-up of a CG head."