Dubai's 829.8 metre high Burj Khalifa stands head and shoulders above any other man-made skyscraper in the world. But it would be positively pint-sized if some of the following super-tall structures ever make it off the drawing board.

The world's tallest proposed buildings

X-Seed 4000

• Height: 4,000 metres
• Floors: 800
• Year of first Proposal: 1995
• Date of Completion: Undetermined (probably never!)
• Location: Tokyo, Japan

The X-Seed 4000 is quite frankly the Godzilla of mega constructions. This monster Japanese structure was the vision of Taisei Corporation back in the mid-nineties and is the tallest building ever fully envisioned. How about these for vital statistics; the Tokyo structure would measure four kilometres (2.5 miles) high, comprising 800 floors, with a sea-base 6 km wide (3.7 miles).

If the build ever got off the ground it could accommodate a population of anything between 500,000 and 1 million inhabitants. It's likely the project was a canny publicity stunt and was never seriously intended for construction – particularly with a top-end price tag of $1 trillion. Brilliantly, the Mount Fuji shaped tower would be more than 200 metres taller than the iconic mountain it's modelled upon and would experience a range of pressure and weather fluctuations because of the enormous elevation.

Ultima Tower

• Height: 3,217 metres
• Floors: 500
• Year of first Proposal: 1991
• Date of Completion: Undetermined
• Location: San Francisco, USA

American architect Eugene Tsui does not do things by halves. Actually compared to those Taisei trailblazers he does. But at two miles high his hypothetical super-tall skyscraper is no slouch. The 140 million square feet of floor space could comfortably house 1 million occupants, as part of a utopian response to overpopulation. Environmental credentials have not been overlooked either with an ingenious system to generate electricity via the atmospheric pressure variance between the top and bottom of the tower.

Dubai City Tower

• Height: 2,400 metres
• Floors: 400
• Year of first Proposal: 2008
• Date of Completion: 2025
• Location: Dubai, United Arab Emirates

This beautiful spiralling structure is said to be inspired by the Eiffel Tower...but with a twist, literally. At the dizzying height of 2.4 kilometres high, this behemoth would positively dwarf it's current world-beating neighbour, the Burj Khalifa by a scale of three. That's seven times taller than the comparably pygmy-like Empire State Building.

The central core of the Dubai City Tower would be clad with six outer buildings that connect every hundred floors and a 200 km/h bullet train would act as the principal elevator. Gulp! Renewable energy sources would be used to meet the estimated annual electricity bill of 37,000 Mwh.

Shimizu Mega-City Pyramid

• Height: 2,004 metres
• Floors: Twenty four 30-story skyscrapers!
• Year of First Proposal: 1996
• Date of Completion: Undetermined
• Location: Tokyo, Japan

The Great Pyramid of Giza was a phenomenally ambitious project that's still regarded as a phenomenal feat of human engineering. Fitting then that the Shimizu Mega-City Pyramid draws inspiration from the Egyptian icon for another hypothetical blueprint, earmarked for Tokyo Bay, that's so large it could only be built using carbon nanotube technology that's still being developed.

At 2,000 metres above sea level the pyramid would be 14 times higher than it's ancient predecessor and could house 1 million people. They'd be ferried around on moving walkways and elevators that could be powered by photovoltaic film coating on structural trusses...either that or algae.

Millennium Challenge Tower

• Height: 1,852m
• Floors: Unknown
• Year of First Proposal: 2005
• Date of Completion: Undetermined
• Location: Kuwait City, Kuwait

Also known as the “Al Jaber Tower” this super-tall skyscraper would soar to a height of just under two kilometres. The project's architect, Omero Marchetti's “ethic city” concept would do away with traditional cast iron and concrete materials, as well as right angles and perpendicular planes. He would prefer to reach the magical nautical mile of height using lightweight structural supports that mimic the “hexagonal matrices of snowflakes.” As you do.