Kiev, Ukraine (CNN) — On the outskirts of Kiev, somewhere between the city's Nyvky and Sviatoshyn metro stations, sits a drab industrial building that you could drive past a thousand times without guessing it contains an extraordinary secret.

Inside can be found the unfinished chapter of one of the greatest feats of Soviet aviation ever conceived. The only clue is the building's size. It's gargantuan.

It needs to be. Because it contains something equally vast -- the largest airplane that was never completed.

The aircraft is an Antonov An-225, conceived by Soviet engineers in the dying days of the Cold War as a gigantic, gravity-defying workhorse that would help communism's ongoing race into space and assert the East's dominance of the skies.

The Antonov AN-225 is the biggest airplane in the world. Mriya's wingspan measures 290 feet, that's longer than five semi-truck trailers set end-to-end.

Only one An-225 was ever built by the Kiev-based Antonov company, which came up with the design. Romantically named Mriya, (Ukranian for dream), it first took flight in 1988 and has been in service ever since, drawing crowds of admirers wherever it spreads its huge wings.

Construction was begun on a second plane, a sister for this aerial leviathan. But while Mriya is breaking world records in the skies, her twin still lies in pieces, only able to dream about leaving the ground.

The fate of Mriya's hidden sister is a fascinating story about big ambitions and even bigger frustrations caught up in the turbulent history of modern Ukraine after the collapse of the Soviet Union.

The story isn't over though. Antonov remains optimistic it'll get the second An-225 off the ground.

It recently granted CNN Travel an exclusive tour of the half-built aircraft, an intriuging glimpse at the legacy of one of the marvels of the modern aviation world.

Cathedral of mechanics

The An-255 is stored in a huge hangar alongside other semi-built aircraft. Pavlo Fedykovych

Reaching the unfinished An-225's hangar involves being escorted by car through the vast industrial landscape west of Kiev that Antonov occupies. Entering the building is like stepping into a cathedral of mechanics -- it's surprisingly calm and tranquil.

The cavernous, endless space swallows up the machinery and airplane parts within. Workers can occasionally be glimpsed in the distance, but the sound of their activity is lost, absorbed by the giant metallic structure.

Towering over everything is the massive fuselage of the unfinished An-225. It's a beast of a thing. If ever completed, it will have a length of 84 meters (276 feet) -- a whole 9 meters longer than the world's largest passenger aircraft, the Airbus A380 superjumbo.

It's an impressive sight, although it is slightly depressing to see this potentially majestic airplane in pieces. The wings that would give it a span of 88.4 meters are unattached, stretching off to one side. The nose gear, a mechanism the size of a house, is also nearby.

So how did it get here?

The story of the An-225 begins back in the 1960 and '70s when the Soviet Union was locked in a race into space with the United States.

By the end of the 1970s, the need arose for transporting large and heavy loads from their places of assembly to the Baikonur Cosmodrome, the sprawling spaceport in the deserts of Kazakhstan that was the launchpad for Yuri Gagarin's pioneering space voyage of 1961.

The cargo in question was the Buran spacecraft, the Soviet Union's answer to NASA's Space Shuttle. Since there were at the time no airplanes capable of carrying it, the Antonov company was ordered to develop one.

What emerged was the An-225 megaplane -- the biggest and most powerful airplane ever to successfully enter service. And on December 21, 1988, three years after she was first conceived, Mriya safely transported the Buran spacecraft to Baikonur.

'Elephant dance'

Mriya was built to carry Soviet space shuttle Buran to its launchpad in Kazakhstan. SVF2/Universal Images Group Editorial/UIG via Getty Images

To this day, Mriya remains the heaviest aircraft ever built. Powered by six turbofan engines, she has a maximum payload weight of 250 tonnes, which can be carried inside or on its back. It boasts the largest wingspan of any airplane in operational service.

Because of its size, pilots need special training to cope with the challenges of maneuvering the An-225. One of the airplane's quirks is its ability to perform a so-called "elephant dance," a term used in aviation when the nose gear "kneels" to make cargo loading easier.

With Mriya declared a success, the Soviet Union forged ahead with plans to build three more An-225s. Construction of the second began in 1989 amid equally high expectations.

Then history intervened.

In 1991 the Soviet Union collapsed, taking with it the Soviet space program. In the chaos that followed, production continued on the second plane, but it was eventually halted in 1994.

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While its manufacturer Antonov successfully transitioned from communism to capitalism, the end of Soviet funding for the ambitious megaplane project meant the unfinished aircraft was in limbo.

The changing geopolitical landscape meant that Mriya was no longer relevant. With the technological rivalry of the two global superpowers ending abruptly, the race to build bigger and more powerful engineering status symbols was at an end.

The An-225's impressive capabilities suddenly were deemed excessive for the modern aviation world -- and certainly one gargantuan aircraft was enough. With very few oversized payloads needing transportation, another Antonov, the 150-tonne capacity An-124 Ruslan, was doing most of the work.

Furthermore, when Ukraine was plunged into a revolution in 2014 that set it at odds with Russia, it lost a key supplier of parts and equipment, putting another question mark over the second An-225's future.

Jigsaw puzzle

Antonov says the second An-225 is 70% complete. Pavlo Fedykovych

Antonov, however, says finishing the build should be relatively simple.

Growing private sector interest in space exploration, tourism and communications -- and the prospect of heavy payloads in need of transportation -- may yet decide the aircraft's fate.

"When there is a need to solve such a problem, there will be a demand for the completion of the second aircraft and the investors will appear," says Gennadiy Silchenko, Antonov's An-225 program director.

Today, the second An-225 is about 70% completed. All the essential components of its superstructure have been manufactured, including the fuselage, wings, nose gear and tail.

Surveying the giant jigsaw puzzle of airplane parts, Silchenko insists they could be quickly assembled should sufficient funding -- between $250 million and $350 million -- arrive.

Once the investment is in, he says, the existing parts will be connected, the control panel developed and the horizontal stabilizer finished. Then the second An-225 would be ready for conquering the skies.

Because it's been kept in a state of conservation, Silchenko adds, the completed aircraft will be as-new, with no limitations on its capabilities.

Assembly nearly happened in 2016, when China expressed an interest in completing the construction, but because of difficulties of transporting the aircraft parts to Chinese soil, it never happened.

Silchenko says that while the company is still open for different options, the aircraft could be successfully put together and completed only in Kiev.

Should it ever leave Kiev, the second An-225 would certainly cause a sensation -- if the adulation of its sister aircraft is anything to go by.

Cult following

The completed An-225 attracts crowds wherever it flies. Greg Wood/AFP/Getty Images

Because of its design and size, Mriya has a cult following among plane lovers who frequently gather to see it land and take off during commercial flights.

A crowd of more than 15,000 spectators came to Perth Airport in western Australia to witness the plane arrive during a visit in May 2016.

Among reasons for its popularity are the mind-blowing 240 world records the An-225 holds, including transportation of the heaviest commercial cargo and carrying the largest single piece of cargo.

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It's also won hearts for participating in humanitarian operations. In 2010, it transported 110 tonnes of equipment and supplies to the Dominican Republic to help with relief efforts in neighboring Haiti after a devastating earthquake.

Needless to say, Mriya has also come to the attention of Hollywood. A CGI-generated aircraft inspired by the An-225 appeared in sci-fi apocalypse movie "2012." A similar rendering starred alongside Vin Diesel in "Fast and Furious 6." The airplane also served as the inspiration for Jetstorm, a shape-shifting robot in the 2007 "Transformers" film.