Also called "dikengyuan" in Mandarin, which means "pit courtyards," the homes are a type of cave dwelling ("yaodong") commonly found in the Loess Plateau in northern China.

"Academic literature mentions they came into existence approximately 7,000 years ago," Lim Tai Wei, an adjunct senior research fellow at the East Asian Institute at the National University of Singapore, told Insider.

"They are considered a type of folk house adaptation to the natural terrain in that part of China," he said.

More importantly, the yellow earth of the Loess is not only soft and easy to dig into, but also sturdy enough that it holds together even without support.

"Some scholars outside China consider the dwellings to be a natural adaptation to the climatic patterns in that region, with harsh and lengthy winters, and extremely burning summer heat," Lim said.

While the size of the courtyard varies, they can go up to 12 meters (39 feet) long in plan.

After digging a pit for the courtyard, which can range between 6 meters to 10 meters (20 feet to 33 feet) deep, the rooms are then carved out from the four walls of the sunken courtyard.

"The rooms may also be reinforced with stones and clay walls or columns," he said.

The houses are then clustered into sunken villages where one cannot see the physical buildings from above ground, he added.

A ramp is also cut into the sides of the courtyard to allow residents to move easily from their homes to the level ground above.

"While millions of people lived in these pit houses at one point of history, it is unclear how many can fit into a room or a home, given that there are individual variations on how they share the spaces," he added.

Many of these ancient homes, especially in the Shanzhou district of Sanmenxia City in China's Henan province, have been restored and converted into tourist attractions.

"Some developers even built modern hotels there," Lim said.

The modernized homes tend to be reinforced with concrete and some are even equipped with solar panels, Lim said.

The exact location the photo above was taken in is unclear, but it shows that some cave homes manage to fit sleeping, living, and kitchen areas into close quarters.

The cave dwellings of Yan'an in the Shaanxi province of China, for example, are famous for having housed Mao Zedong and his political allies in the years leading up to China's Communist revolution.