Over the years, dozens of new theme parks have been proposed, but never built. Some of these were almost impossibly ambitious - but came surprisingly close to becoming a reality.

Five floors of rides housed inside an abandoned power station. A full-scale recreation of a fictional spaceship. A cage that lowered guests into a pool full of man-eating sharks. All of these too-crazy-to-be-true attractions were really planned - and some of them even got as far as initial construction work. But, sadly, they were not to be.

Take a look at the concept artwork for 5 incredible theme parks that failed at the last financial hurdle, and dream of what might have been.

4. DisneySea

Following the success of Walt Disney World, Disney was keen to transform Disneyland into a similar multi-day destination resort. The problem, though, was land – it didn’t have much of it to work with close to Disneyland. Instead, it proposed building a new theme park in Long Beach, which it dubbed DisneySea and announced in 1990. It was to be the most ambitious Disney theme park ever, sprawling across 225 acres.

The entire park would have featured a marine theme, and would have been accompanied by no fewer than 5 hotels, a cruise ship port, a marina and an evening entertainment area. Disney had already acquired the RMS Queen Mary (which would have become one of the hotels) and the Spruce Goose, which would have been incorporated into the new resort.

Similar to a SeaWorld park (but with much heavier theming), DisneySea would have combined live animal exhibits with high-tech theme park attractions. It was to feature lands themed around a Grecian village, an Asian water market and a Caribbean lagoon. Each was to be themed around a real or fictional port.

The most ambitious element, though, was Oceana, a stunning, multi-domed structure that would have been the world's largest aquarium. This would have featured tidal exchange with actual ocean, so as the tide changed, the levels of water in the outside display tanks would rise and fall.

The aquarium would have held a ridiculous 10 to 12 million gallons of water, making it double the size of the one at Epcot's Living Seas pavilion (itself the world's largest when it opened). Oceana would also have hosted a real, working research center, bringing together scientists from around the world.

Elsewhere, City of Atlantis would be an underwater trip to the mythical lost city, Pirate Island would be a child-friendly area for exploration and Nemo's Lava Cruiser would simulate a trip into underground caverns. In the Adventure Reef area, guests would have been lowered in a steel cage into a tank full of sharks.

So what went wrong? Disney thought that the EuroDisney project was a slam dunk. Instead, when it opened in 1992 it immediately began to haemorrhage money. CEO Michael Eisner lost his enthusiasm for the DisneySea project, and it was ultimately dropped. Disneyland visitors ended up with the underwhelming Disney's California Adventure instead.