The United Nations says that the world population will quintuple by 2300.
This means cities are going to get very crowded. More than half of humanity already lives in cities, and by 2050, at least two-thirds of the population will call cities home.
One solution: Building underground or into the existing landscape, whether that's a hill or a cliff.
Lots of cities already have subterranean spaces, but as urban areas become more crammed, you can expect these spaces to multiply.
Take a look at some of the world's most unique marvels that are built into the ground and cliffs.
This means cities are going to get very crowded. More than half of humanity already lives in cities, and by 2050, at least two-thirds of the population will call cities home.
One solution: Building underground or into the existing landscape, whether that's a hill or a cliff.
Lots of cities already have subterranean spaces, but as urban areas become more crammed, you can expect these spaces to multiply.
Take a look at some of the world's most unique marvels that are built into the ground and cliffs.
SANTORINI HOTEL, GREECE
The Iconic Santorini Hotel in Imerovigli, Greece, has a pool that's embedded inside a cave. The cliffside overlooks the Mediterranean ocean.
LES HAUTES ROCHES HOTEL, FRANCE
For a more modern take on a troglodyte dwelling, Les Hautes Roches is a five-star hotel constructed inside a limestone cliff. The lavishly decorated rooms overlook the Loire River.
SALINA TURDA, ROMANIA
This 400-foot-deep amusement park was constructed in a former salt mine in Romania, called the Salina Turda. Inside, you can ride an underground ferris wheel, paddle in a lake, play ping pong, and see performances at an amphitheater.
TEMPPELIAUKIO CHURCH, FINLAND
Built into subterranean rock, the Temppeliaukio Church in Helsinki boasts a glazed dome just above ground level that lets ample sunlight into the interior. Those rough rock walls were left untouched by the designers for a reason: naturally great acoustics make the church a perfect venue for concerts.
NAOSHIMA, JAPAN
This "buried" museum is on Naoshima, Japan's "art island." Small concrete openings and geometrical skylights dot the greenery.
POLAND
In Poland, 800 steps lead down a shaft to this space, which holds an art gallery, health resort, and massive halls that host weddings and conferences. This 13-century marvel was once a salt mine.
SERBIAN ORTHODOX CHURCH, AUSTRALIA
The residents of Coober Pedy in Australia move underground to pray. The Serbian Orthodox Church, built in 1993, is carved in the sandstone and has a community hall, a parish house, and even a school.
CAPPADOCIA, TURKEY
Rocky homes line the cliff face in Nevsehir, Turkey, also known as the Cappadocia region. The area is popular with tourists for its Byzantine art and large network of underground Bronze Age troglodyte dwellings.