Hong Kong, to put it simply, is running out of room. More than 7 million people live in the 427 square mile autonomous region of the People’s Republic of China. Country parks take up 41 percent of the land mass, all of which is dotted by steep hills, making it difficult to build anywhere. Many high rises have maxed out on height restrictions. Meanwhile, housing prices hit record highs this spring: Hong Kongers are paying nearly $1,500 per square foot. The average home costs $1.8 million.

The government’s looking into rezoning and building in rural areas, but it’s also in search of a long-term strategy to create more real estate. What to do? Well, according to the geotechnical engineers in the Hong Kong Special Administrative Region’s Civil Engineering and Development Department, the answer is down. Like, under the ground, in newly built caverns.

“All the urban flat land in Hong Kong is already a built-up area,” says Tony Ho, the department’s chief geotechnical engineer. Since the early 1980s, the Hong Kong government has explored the idea of building caverns, straight into hilly areas too, well, hilly to develop. In 2017, the government completed a comprehensive feasibly study, pinpointing 48 prospective caverns for long-term development, ranging in size from 0.1 to 0.8 square miles. Six more studies to push the project along are already underway.1

This month, the project walked away from the annual International Tunneling Association’s awards banquet—in gay Paris!—with the award for Innovative Underground Space Concept of the Year. (It beat out an Israeli plan to use tunnels to create an underground cemetery and a Turkish transportation megaproject.)

No, the plan is not to make people live like bats in these cool, dark spaces. Instead, the newly created real estate could host sewage and water treatment plants, data centers, reservoirs, and storage facilities. The government is considering other (lower priority) options, too: archives, oil, gas, or wine storage, bicycle and car parking, laboratories, and sports facilities. The caverns could hold mausoleums, mortuaries, incinerators, crematoriums, and, to round out the death theme, slaughterhouses.

“What we are thinking is, if we can best use the underground space resources, we can turn the constraint into an opportunity,” says Ho.

Using tunnels to create extra space is far from a new idea. The city’s engineers cite inspirations in Norway (the underground Gjøvik Olympic Cavern Hall, built in 1975 but updated in 1995, can host 5,500 people in its pool and ice hockey facility); Singapore (it finished its Underground Ammunition Facility in 2008); and good ol’ Kansas, USA (entrepreneurs there transformed a limestone quarry into an underground industrial park used for cloud computing storage, packaging manufacture, and sorting mail.) Hong Kong itself already has some rock cavern facilities, including a 424,000 cubic foot salt water reservoir for Hong Kong University, a sewage treatment works completed in 1995, and an explosives depot. A waste transfer facility refurbished in 2013 marks the city's biggest rock cavern project to date.

But the work going on now is grander in scale. It's a scheme to totally reorganize public space in the region. “When we try to relocate facilities underground, that allows an opportunity to replan a whole area,” says Edward Lo, chief town planner with the Hong Kong Planning Department. Moving functions like parking and data storage under the earth means that land can be used for housing or businesses, and hopefully ease prices.

Big plans don’t come cheap. The Hong Kong government estimates rock cavern excavation alone can range from $190 to $250 per cubic foot; total cost of construction might be $320 to $450 per cubic foot. One very small, 12-car parking garage could cost $7.3 million to construct.