Forget skyscrapers - the world's most beautiful buildings are underground

The United Nations says that the world population will quintuple by 2300.

That means cities are about 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.

In major cities like San Francisco, the demand for housing is outpacing the ability to supply houses. But that growing population will need places to stay.

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.

Shivam Saini contributed to an earlier version of this story.