5. The Seas aquarium

What it is: An enormous saltwater aquarium that holds 5.7 million gallons of water, located in The Seas with Nemo & Friends Pavilion at Epcot.

How it was built: An enormous budget of $90-million made The Living Seas Pavilion the most expensive attraction ever built at a Disney park when it opened in 1986. It took 22 months to build, with 12,000 cubic yards of concrete and 900 tons of reinforcing steel being used to build the tank. The rest of the pavilion consumed 8,000 cubic yards of conrete and 850 tons of structural steel. The recipe for the artificial saltwater called for 27 truckloads of common table salt. Fun fact:The aquarium is so huge that Spaceship Earth would fit inside it.

4. The Utilidors

Image: Disney

What it is: An enormous network of tunnels underneath the Magic Kingdom, used for utilities and to enable Cast Members to move around the park without being seen by guests.

How it was built: The 15-foot-high concrete utilidors were built first. They were then covered over using soil excavated from the man-made Seven Seas Lagoon in front of the park. The Magic Kingdom's public walkways and attractions therefore sit above ground level. Fun fact:Until 2005, the costuming department - with over 1.2 million outfits - was housed in this underground lair.

3. Tree of Life

Image: Disney

What it is: An enormous 145-foot-tall, 50-foot-wide tree at the heart of Disney's Animal Kingdom. It houses the It's Tough to be a Bug show.

How it was built: In an ironic twist for an "environmentally-friendly" park, the Tree of Life is based around an old oil rig. This was wide enough to house the attraction's theater, and strong enough to hold around 8,000 branches. These host 103,000 mock leaves, each of which was individually placed. It took 10 artists and three Imagineers around 18 months to create the 325 animals carved into the tree. The sculptors had around 6-10 hours to produce the final image before the plaster hardened. Fun fact:There are more than 4 million living plants at Disney's Animal Kingdom - very few are fake like the Tree of Life.

2. The Forbidden Mountain

What it is: The huge mock mountain that towers over Asia in Disney's Animal Kingdom, housing the Expedition Everest roller coaster.

How it was built:The Forbidden Mountain was created using 5,000 tons of structural steel and 10,000 tons of concrete. A rigid steel structure holds the mountain in place. More than 2,000 gallons of stain and paint were used on the mountain's rockwork and the buildings in the surrounding village. To recreate the lowlands surrounding Mount Everest, more than 900 bamboo plants, 10 species of trees and 110 species of shrubs were planted.

Image: Raul 654, Wikimedia (license)

Fun fact:At 199 feet tall the Forbidden Mountain is the tallest of Walt Disney World's faux mountains. It is just below the Federal Aviation Authority's 200-foot limit, which would require it to have a flashing red light on its peak.

1. Hollywood Tower Hotel

What it is: The enormous fake hotel that houses the Twilight Zone Tower of Terror ride at Disney's Hollywood Studios.

How it was built: The design of the hotel, which is meant to evoke the golden age of Hollywood, was inspired by The Mission Inn in Riverside, California. Some 1,500 tons of steel were used to construct it, along with 145,800 cubic feet of concrete. The roof is lined by 27,000 tiles. The drop sequence is powered by two huge motors, which are 12 feet tall, 35 feet long and weigh a massive 132,000 pounds. Fun fact: In his autobiography, former Disney CEO Michael Eisner reveals that he suggested that the Hollywood Tower Hotel be an actual hotel, as well as hosting the Tower of Terror. This proved to be impractical.