By Rina Chandran
HONG KONG - When authorities drew up a plan to make Hong Kong a hub for Asia's wine trade, they faced a big challenge: where to store the bottles in a city that was fast running out of space. So they went underground.
Officials presented World War Two-era bunkers as potential sites, and former diplomat Gregory De'eb and businessman Jim Thompson signed a lease on them, setting up Hong Kong's first commercial wine cellar 20 metres (66 ft) below ground in 2003.
Crown Wine Cellars can store more than 100,000 bottles, and also has a subterranean clubhouse.
"It is the underground aspect of the club that is its most attractive and popular feature," said De'eb. "Members even view the lack of mobile phone reception as overwhelmingly positive."
Hong Kong plans to move more facilities beneath the Earth's surface to free up space above, in one of the world's priciest real-estate markets.
With almost 70% of the global population expected to be living in urban areas by 2050, according to the United Nations, cities are coming under the spotlight as never before.
From Singapore to sub-Saharan Africa, they are fast running out of space to house their swelling populations.
Cities have long put metro rail networks, as well as utilities like sewage and water pipes underground, with several also moving commercial, retail and storage facilities down below to free up space or better handle extreme temperatures.
In Hong Kong, known for its towering skyscrapers and wooded hills, sky-high home prices have boosted the urgency of maximising use of underground space.
The government has vowed to create more land for housing, including by building artificial islands.
It is also looking to use underground space for waste treatment, data centres, water reservoirs, power stations, crematoriums and sports facilities.
The city's rocky terrain lends itself to cavern development as a "cost-effective alternative" for long-term land supply that offers safety, environmental and security benefits, said Edward Lo, Hong Kong's chief town planner.
"Given the lack of land resources in Hong Kong, it has all along been our policy objective to develop underground space," he added.
From the catacombs of ancient Rome to step wells in medieval India and army bunkers, underground spaces have been used for a variety of reasons down the ages.
Helsinki and Montreal, which are blanketed in snow for several months of the year, are considered leaders in "underground urbanism", a movement focused on innovative ways to use underground space.
Besides mass transit, growing concerns around the environmental and health impacts of "bad neighbour" facilities such as refuse transfer and waste treatment plants have pushed cities to consider moving those below ground as well.
Underground space is ideal for a densely populated city like Hong Kong, which has more than 7 million people crammed into an area of about 1,100 square kilometres (425 square miles), with less than a quarter of that land available for development.
About a decade ago, authorities unveiled a policy aimed at studying the opportunities in underground space.
In a government-commissioned study, consultancy Arup identified 48 potential underground and hillside sites for new caverns, and some 400 state facilities that could be moved underground.
"The idea was to better integrate facilities, so people can easily move between them, and avoid the conflicts of traffic and weather disruptions above ground," said Mark Wallace, director of infrastructure at consultancy Arup.
"For the city, it results in more efficient use of space, reduces the impacts of urban sprawl, and helps preserve the natural environment," he told the Thomson Reuters Foundation.
While excavation and building underground are more expensive, there are savings on maintenance and land costs, he added.
And less energy is needed because of more stable temperatures underground, an important factor for cities looking to curb their carbon emissions.
Underground structures also perform better in earthquakes and need fewer repairs, while offering better protection from typhoons and thunderstorms, which are forecast to become more severe as global temperatures rise.
Cheaper, fast-digging technologies such as those used by billionaire entrepreneur Elon Musk to build a high-speed transport tunnel in Los Angeles, meanwhile, can cut construction time and minimise disruption above ground.
"As older urban areas in Hong Kong deteriorate and new infrastructure or redevelopment is needed, underground development is a way to build new facilities with minimum disruption to the surface and public," Wallace said.
Elsewhere in Asia, space constraints and security concerns prompted the development of underground space decades ago in Japan and South Korea.
More than 20 Chinese cities including Beijing, Shanghai and Shenzhen are now making plans for urban underground space, while Singapore unveiled an underground master-plan this year.
Hong Kong, like Singapore, has traditionally relied on reclaimed land, but that is seen as increasingly unsustainable because it fuels environmentally harmful processes like sand mining, said Mee Kam Ng, director of the urban studies programme at The Chinese University of Hong Kong.
Digging out space underground can also have effects on groundwater and surface ecology, she said.
Instead, Hong Kong could free up nearly 4,000 hectares through better planning for its existing land resources such as the largely rural New Territories in the north, she said, citing research by the Citizens Task Force, a non-profit network.
Some residents are resistant to the idea of tearing up parks to create shopping malls underground.
An ongoing study of potential underground development in Tsim Sha Tsui has drawn criticism from planners and locals, who say any benefits will be outweighed by the damage to Kowloon Park, a green oasis in the congested city.
The plan proposes that underground spaces be built beneath about a quarter of the 13-hectare park for retail and community facilities, parking and pedestrian passages.
Areas of the park could be affected for up to seven years during construction, while parts of it will be surrendered for access points, opponents say.
Authorities say the development will improve pedestrian movement and reduce congestion, and that "old and valuable trees" will be preserved.
But Paul Zimmerman, chief executive of Designing Hong Kong, an urban think-tank, said the project would wreck the park, while failing to ease traffic significantly.
"Good use of underground space is definitely needed in Hong Kong, and it makes sense in some cases - like roads and sewage treatment, and wine cellars," he said.
"But it does not make sense in others, like building a multi-storey complex under a park that will destroy the park's character and only creates more shops and parking."