In 1931, construction workers added the final nail to the Empire State Building in New York. At 1,250 feet high, it was the tallest building in the world, and remained so until the 110-story north tower of the World Trade Center finalized in 1972.
Then came Chicago's Sears (now Willis) Tower, followed by the Petronas Twin Towers in Kuala Lumpur and the Taipei 101 in Taiwan. In 2010, Dubai completed construction on the massive, 2,722-foot-high Burj Khalifa.
See also: 5 Ways Cities Are Using Big Data
The 20th century was inarguably the era of the skyscraper. Cities across the world, out of necessity and sheer showmanship, expanded up, up, up. But the 21st century is seeing a new trend of going underground instead. Urban areas such as Helsinki and Paris are looking to expand below the surface for resource, retail and travel purposes.
Video: YouTube, AVsaha
One of the main reasons cities are digging below the surface is climate change, according to Clara Irazábal, Ph.D., assistant professor of urban planning and architecture at Columbia University.
"In New York, especially, we've received extraordinary weather over the past few years. And that's probably going to become the norm," she says. "It makes sense to preserve the economic vitality of the city to have underground levels, particularly ones that will make traveling as pedestrians or even cars safer during storms above ground."
In cities with high pollution levels, like Hong Kong and Beijing, underground complexes could make for healthier commutes to work.
It would be easy to control temperature and ventilation levels underground as well, Irazábal says, so it's especially appealing to areas with traditionally cold winters or scorching summers: "This is a trend we should pay more attention to. I think we'll be seeing more of this over the upcoming years."
We rounded four projects that cities are working on across the world. Some are in their planning stages, while others are well underway.
In an effort to fight cold temperatures and maintain its low-rise city skyline, Finland's capital, Helsinki, began constructing an underground network of public and resource spaces in 2011.
The city refers to it as an "Underground Master Plan," and as of 2014, engineers have built more than 400 premises, ranging from aquatic facilities and shopping malls to storage spaces for oil and coal, all more than 30 meters below the surface. According to the city's website, there are more than 200 longterm projects scheduled for the future.
Video: YouTube, Ruben Alonso
Underground pedestrian paths connect the main points of the city, and they're accessible by various stairways throughout town, much like the entrances to standard subway systems. It's a blend of old-versus-new, too. Underneath the Uspenski Cathedral, which was built in 1862, is a new data center for the city.
The ultimate goal is to expand the network under the entire Helsinki region, clearing up space on the surface for real estate and residency while hiding the industrial components underground. But having a warm commute under the surface on a chilly February morning doesn't hurt, either.
Paris: The City of (fluorescent underground) Light.
Parisian mayoral candidate Nathalie Koziuscot-Morizet, who, if elected, would be the first female to hold the position, has released her plans to reinvent some of the city's abandoned metro stations.
Architects Manal Rachdi and Nicolas Laisné threw together a series of sketches that show a swimming pool, nightclub, theater, restaurant and art gallery taking over a long-dormant train tunnel.
One of the stations, called "Haxo," currently looks like this:
Here are a few other sketches that the architects released:
Most of the "ghost stations," as they're called, have been abandoned for more than 75 years. Some were shut down due to lack of passenger use; others never saw an actual train after planning was scrapped.
The team is eyeing a total of eight metro stops across the city, including Champ de Mars, Arsenal and Saint Martin et Martin Nadaudso. More ideas are likely to come. The elections begin March 23.
Montreal's RÉSO is a vast underground network of 33 kilometers of pedestrian walkways beneath the city. Unlike Helsinki's method of hiding industrial departments below the surface, Montreal's underground complex is more commercial-based, with 40 entertainment venues and attractions.
Trains and buses take commuters to and from their destinations, helping to give it the appearance of a full, underground city. There are more than 120 exterior access points to the complex. More than 500,000 residents use the network every day, either to visit its stores or just utilize its shelter during cold days.
4. New York
A group in New York has proposed plans to transform an abandoned trolley terminal into an underground park. The "Lowline," a name chosen to complement the city's High Line park, would transform a one-acre tunnel below Manhattan's Lower East Side into a public space. A solar collection dish, situated above ground, would gather sunlight and transfer it through a tube down to dome lights above the underground park.
The project gained its initial funds through a Kickstarter campaign in 2012. Dan Barasch, one of the project's cofounders, tells Mashable that every Lower East Side elected official has supported the idea. Logistics are still being finalized, he says, but he expects the park to be completed in five years.