Space-strapped cities like New York and San Francisco may think they have it rough, but the booming Hong Kong may have an even more immediate problem.

The population surge is making real estate developers consider becoming cave-builders.

The Hong Kong government has begun searching for a long-term solution, one that may include building underground for non-residential purposes, according to a report in Wired. Hong Kong officials have considered cavern excavation since the 1980s, following in the footsteps of engineers for projects in Norway and Kansas.

Already one of the most expensive cities in the world, Hong Kong is one of the most densely populated places on the globe, and is simply running out of space as its headcount jumps further above 7 million. A booming economy has led to commensurate economic growth, with 2017 growth expected to check in around 3.7 percent.

Developers are struggling to house key sectors that help the economy run. Additionally, they are being forced to navigate stiff building restrictions that make it harder to build more in developed areas.

“All the urban flat land in Hong Kong is already a built-up area,” Tony Ho, chief geotechnical engineer of the Hong Kong Special Administrative region’s Civil Engineering and Development Department told Wired.

However, rezoning and building in rural areas could provide some temporary relief, the report noted. The government is exploring options to make more space for housing above ground by relocating facilities — like water and sewage treatment plants, data centers and reservoirs — underground.

“What we are thinking is, if we can best use the underground space resources, we can turn the constraint into an opportunity,” Ho told Wired. Longer-term, the project could redefine public spaces in the region.

The government has already earmarked funds for future projects and kicked off feasibility studies. In 2017, the government conducted a study, pinpointing 48 prospective caverns for long-term development, and has six more studies underway.

Read more on Hong Kong’s underground city planning on Wired.