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".