Singapore is one of the most built-up cities not only on the surface.
With its ambitious plans to build all kinds of storage and transport infrastructure deep below the surface, Singapore also looks set to be one of the densest underground cities in the world.
Mr Tan Boon Khai, chief executive of Singapore Land Authority, said at a conference in Amsterdam last month that Singapore plans to intensify land use underground.
"Now that we're almost fully built up on land, we have to plan and build underground," Mr Tan said during a panel discussion on underground maps at the Geospatial World Forum 2019, which The Straits Times attended.
Singapore laws, which limit land ownership to a maximum of 30m below the surface, will open up many opportunities for huge infrastructure projects from rail networks to reservoirs to storage facilities.
This is because new developments can run deep below private property and need not be held back by these ownership rights. "Anything deeper is owned by the state," said Mr Tan.
More importantly, the move down will free up precious space above ground to build a more liveable city.
"We want to dig deeper to build reservoirs, roads and storage facilities so that the ground above can actually be freed up for homes and parks," Mr Tan said.
For instance, Singapore's first 230kV underground substation, to be located next to Labrador Park MRT station, will free up 3ha of land above ground for development.
FREEING UP SPACE ABOVE GROUND
We want to dig deeper to build reservoirs, roads and storage facilities so that the ground above can actually be freed up for homes and parks.
MR TAN BOON KHAI, chief executive of the Singapore Land Authority.
The Jurong Rock Caverns, a storage facility for liquid hydrocarbon 130m under Jurong Island, has freed up more than 60ha - the equivalent of 84 football fields - of development space on the island.
Accurate maps are crucial to such developments. To provide the nation's first comprehensive 3D look at what lies up to hundreds of metres underground, the Urban Redevelopment Authority (URA) released a subterranean masterplan in March this year.
Maps for Marina Bay, Jurong and Punggol were the first to be rolled out, marking the first of many steps in building a seamless underground map of the country. More areas will be added over time.
The maps show specific locations of bus and rail networks, carparks, pedestrian links, roads, logistics use and utilities, up to a depth of 8m for Jurong, 15m for Punggol and 25m for Marina Bay.
At the launch of the URA subterranean masterplan, National Development Minister Lawrence Wong said that Singapore's underground maps "will help coordinate planning of infrastructure, storage and utilities".
The tight squeeze on land has also been felt by other nations, which have started creating comprehensive 3D subterranean maps.
The Netherlands, for one, needs to build one million additional homes in the cities.
"We are struggling to find space," said Mr Martin Peersmann, programme manager for the Dutch Basis Registratie Ondergrond (BRO), or Key Registry for the Subsurface. He was speaking at the Geospatial World Forum 2019.
The BRO is an open database backed by legislation that requires any firm that excavates or drills underground to share what it discovers about soil and groundwater with the BRO registry. Any user who has found inaccurate information is also required to report it.
The database will go towards the creation of an authoritative and comprehensive 3D underground map to help the Netherlands to tackle the effects of rising sea levels and geological changes.
For instance, engineers can determine how rivers have shifted their course and how soil conditions have changed over time, ahead of laying or reinforcing the foundations for bridges, buildings and dikes.
Larger sewers to cope with a growing population can also be better planned.
Other cities, such as Montreal in Canada and Helsinki in Finland, have also created extensive underground networks.
Montreal's Reso, commonly referred to as the Underground City, comprises 32km of tunnels covering about 12 sq km in the most densely populated part of Montreal.
The tunnels connect train stations, office towers, hotels, shopping centres, residential and commercial complexes, convention halls, universities and performing arts venues.
The underground network is particularly useful during Montreal's long winters, during which over half a million people use it every day.
McGill University, Montreal Museum of Fine Arts, Concordia University and University of Montreal also have campus tunnel networks separate from the Underground City.
Helsinki's sprawling underground - comprising 500 separate underground facilities and 300km of tunnels - is primarily designed to offer shelter from Russia's potential threat.
For instance, its advanced air and water filtration systems, toilets, beds and food supplies could house people underground continuously for up to 14 days. It is designed to shelter 750,000 people, more than the entire population of the city above ground.
Since the 1960s, the city has excavated nearly 9 million cubic metres of the hard granite bedrock that it sits on.
It is all part of Helsinki's Underground Master Plan, with most of the areas - including carparks, shopping malls, swimming pools and railway tunnels - for public use.
30m below surface is measured from the standard Singapore height reference mark.