Gone underground: train tunnels, a garbage compact centre, a sewage treatment plant and an explosives depot.
Heading down under: a data centre, a government archive and even sports halls.
Burrow deeper into the minds of planning experts and one comes up with visions of an underwater city under Victoria Harbour - a pedestrian corridor with shops and entertainment facilities that connect Hong Kong Island and Kowloon.
There is also a proposal for a subterranean road network under jam-packed Mongkok in Kowloon, so as to free up more ground-level space for shoppers and tourists.
As Mr Samuel Ng, chief geotechnical engineer at the government's Civil Engineering and Development Department, states categorically: "We have to move the city underground; otherwise it's a dead end for Hong Kong."
The mission to go underground has gained even more urgency as the city searches for land to build decent housing for its people, but faces obstacles to developing rural areas from various quarters.
Hong Kong, which has 1,108 sq km of land, has already dug some 400km of underground tunnels for its subway system and water channels. In addition, five rock caverns are being used by the government for the following: a reservoir, a sewage treatment plant, an explosives depot, an MTR station and a garbage compact plant.
There is room underground for more to be done, experts said.
For now, Hong Kong has plans to relocate some 400 government facilities, including civic centres, sports halls and wholesale markets, to rock caverns, which are mainly located in outlying areas. In all, these will yield at least 4 sq km of extra space, said Mr Ng.
There are more ambitious proposals ahead, including for the city centre.
Chief Executive Leung Chun Ying, in his first policy address in January, said his administration "will further explore the potential of developing underground spaces in the urban areas".
A two-year study will be launched at the end of this year. Forty urban areas will be initially selected, before being narrowed down to 15 locations, said Mr Ng.
Mongkok tops his list.
"You cannot even use the term 'wandering on the streets' in Mongkok as it is jam- packed," he said.
Architecture academic Cheung Kwok Pun of Hong Kong University has submitted to the government a proposal for a five-storey basement building under reclaimed land around Victoria Harbour. Totalling 120,000 sq m, it would incorporate offices, a traffic network, carparks and an energy-generating centre. A recreational pedestrian strip could be built beneath the harbour itself.
Such a plan, said Professor Cheung, would help mitigate soaring commercial property prices in premium waterfront areas. At the same time, there will be "synergy" in linking up the financial district of Central with the one coming up in Kowloon East.
However, challenges remain.
A major one is cost. Prof Cheung reckons that it costs twice as much to build underground.
There are also engineering considerations, such as how to keep an underground city well-ventilated and safe from fires.
"If there is a blaze during a football match, what design can ensure that we can safely evacuate hundreds of people above ground?" asked Mr Ng.
Yet another challenge is the tricky issue of land ownership.
Under existing laws, those who own the rights to develop a land parcel also own the rights for what is underground.
"This means that the government has to deal with some private players and figure out how to offer incentives to get their cooperation," said Mr Ng.
He suggested that in future, the government should separate the rights for the two. In Japan, for instance, land that lies 10m or 40m - depending on the situation - under a building foundation belongs to the government.
Additional reporting by Pearl Liu
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