As the number of people in our world keeps growing so do our cities. According to the U.N., just over half of the world's population - about 54% - lives in an urban area. And that number is expected to climb to 66% by 2050.That raises a lot of questions, the least of which is "where are we going to put all those people?"

Well, in recent years, engineers and architects have been trying to design an answer to that. And along the way they've gotten more and more ambitious, designing bigger, taller, more self sustaining buildings that can house more people.

Some of these structures reach straight for the sky, others try to stay more in tune with the environment around them, and some of them draw on ingenious feats of physics.

All of these structures are technically possible to build but most of them never will be, largely because of the cost involved. Still, these mega-structures challenge the very limits of how high we can build, as well as where we can build and how engineering can help tame the forces of nature. And they present some pretty brilliant ideas about how the cities of the future might look.

Chicago is no stranger to tall buildings; of the ten tallest buildings of the U.S., Chicago is home to four. But it could have been home to one that was taller than them all. In 2005 plans were introduced to build "The Chicago Spire." At 610 meters tall it would have been the tallest building in the Western Hemisphere, beating out the Willis Tower, also in Chicago, and One World Trade Center in New York City.

The real genius of the building's design wasn't in how tall it was; it was also in how it was going to beat the wind. Despite being called "The Windy City", Chicago isn't the windiest city in the U.S., in fact it isn't even in the top twenty. But the winds coming off of Lake Michigan could still cause serious problems for a building that's more than a third of a mile tall.

And here, the main challenge is a phenomenon called "Vortex Shedding." When wind hits a tall, flat, rectangular shape, like a skyscraper, it separates into two powerful gusts, with one rushing along either side. These gusts move around the structure and then turn back on themselves, creating huge whirlpools of air. These eddies end up pulling the building back and forth, causing a lot of stress on the structure.

Which is why the Chicago Spire wasn't designed to be a rectangle; instead, it's a helix. The designers have described their building as resembling a "spiral of smoke rising above the Chicago River," but really it looks more like a screw or a giant drill bit.

When the wind hits a helical structure like this, the wind gets caught in its grooves and curved surfaces, creating a whole lot of mini-vortices instead of one huge vortex on each side. The design wouldn't have totally solved the wind problem, so it also would have included twelve concrete walls around the base to provide more stability.

Unfortunately, we may never get to see the Chicago Spire I.R.L. Plans for the tower were scrapped in 2014 after years of delays and major financial problems.

Of course, a helix isn't the only way to combat the wind. Take the Nakheel Tower, a proposed skyscraper in Dubai. At 1000 meters tall and located right on a major waterfront, the Nakheel would have definitely faced high wind speeds.

But rather than trying to direct the wind around the building or break it up into lots of little vortices, the designers came up with a different plan: just let it flow right through. They did this by incorporating vertical slots that ran the length of the tower and by making the building hollow in the middle. This basically created four separate towers standing in a circle, like an extremely tall doughnut cut up into quarters.

With this design, wind could blow from almost any direction and simply flow straight through. For added stability, each of the towers was also connected to the others by way of sky bridges, which had the bonus of making it easier to get from one tower to the other without having to go all the way to the bottom. Construction began on the Nakheel Tower in 2008 but it stopped after a national debt crisis hit Dubai the following year.

Jumping across the world to Jakarta, the capital of Indonesia, there's another proposed mega-tower: The Jakarta Signature Tower. Jakarta has some issues with wind too; when you're dealing with mega-structures, almost every building does. But Indonesia also has a much more pressing issue: earthquakes.

Indonesia is located in the so-called "Ring of Fire," a string of volcanoes and highly seismic areas that surrounds the Pacific Ocean. To make sure this 610 meter tall building could withstand earthquakes the designers had to build in quite a few safety features. For starters, its foundation is made up of a series of concrete cylinders called "piles." Each pile is at least 1.8 meters in diameter and driven down into the ground up to 90 meters deep.

More strikingly, the Signature Tower would also have a kind of exoskeleton: an external frame constructed around the central core of the building, made up of vertical super-columns and horizontal braces called "belt trusses." If the core of the building were to tilt, as it would in an earthquake, then it would puh on some of these external supports while pulling on others. The combined forces of tension and compression competing against each other would end up cancelling each other out so the core would stabilize again.

Construction hasn't begun on the Signature Tower yet but developers say they're planning to have it completed by the early 2020s.

Also located along the Ring of Fire is another proposed tower, the "Sky Mile Tower" in Tokyo. The name of the building says it all: if it were built, it would be just over a mile tall - 1700 meters, to be exact. That's twice as tall as the Burj Khalifa in Dubai.

Since it's located on the Ring of Fire, the design of the Sky Mile Tower definitely has to account for earthquakes. But that isn't the only problem.

The city of Tokyo is only 44 meters above sea level and, as in many coastal cities, engineers have to keep in mind how their designs will weather events like floods and tsunamis, as well as the inevitable rise in sea level that comes with climate change.

So in a kind of counter-intuitive move, designers have proposed building Sky Mile Tower on the water itself, right across the mouth of Tokyo Bay.

It would be constructed in a chain of artificial hexagonal islands anywhere between 500 feet to 5000 feet wide. These six-sided structures would act as breakers, allowing large waves to dissipate against them, rather than against the side of the building. And by situating the tower complex across the mouth of the bay itself, engineers hope that the whole system of buildings would help protect Tokyo from the increased risk of typhoons, storm surges and other severe weather that come with global warming.

Now, because the Sky Mile Tower is so tall, the designers also ran into another problem: how do you pump the water that people need to shower and drink and do all those other human things all the way at the top of the tower. Sky Mile's designers came up with a pretty revolutionary answer: you don't. Instead, residents would get their water from "Cloud Harvesting."

This might sound like something out of science fiction but the idea is actually pretty simple. You probably already know that clouds are made up of tiny water droplets suspended in the air. In order to catch those droplets all you need is a net with small enough holes, something like a strong fabric mesh. When the wind blows clouds or fog through the mesh, water droplets get trapped, roll down into a trough at the bottom, and voilà! a captured cloud.

Sky Mile's plans calls for water collection sites on every few floors, where the captured water would be treated, then it would be delivered to the lower floors using gravity, eliminating the need for pumps.

But as cool as this tower sounds, it will probably be a while, if ever, before we see a mile high skyscraper in Tokyo. The design for the tower and surrounding islands was proposed in 2015 and a lot more testing and planning would need to be done before any construction could start. But even then, no money has ever been devoted to the project.

The fact is, many mega-structure projects that have been drawn up are mostly useful as proposals, as exercises to get people thinking about the engineering challenges of the future and how we can design  new ways to overcome them.

Sure, you might envision a time long from now when you're living above it all, drinking cloud harvested water and sleeping safe in the knowledge that no natural disaster could ever take away your house. If anyone can make that happen, it's engineers. But, for now, it's a dream; one that, like many of these mega-structures, is too awesome to be real.

This episode of SciShow was brought to you by our patrons on Patreon. Thank you so much to all those people who pay for this so that other people who can't pay don't have to. If you want to support us by paying you can go to patreon.com/scishow. If you want to support us by watching that is also fantastic; you can go to youtube.com/scishow to subscribe.