Ever taller buildings used to be the way to go when urban planners ran out of land in growing cities.
Increasingly, they are breaking new ground by digging deeper underground and finding new ways to free up the precious space above.
Overcrowding is eased and the cities become more liveable, not least through ease of travel and more room for open-air recreational facilities.
Singapore is the latest country to join such trailblazers as Japan, Canada, Norway, Germany, France and the Netherlands in exploring the new frontier of smart subterranean cities.
As a first step, the Urban Redevelopment Authority (URA) has released this year a subterranean master plan, providing the first comprehensive three-dimensional (3D) look at what lies up to hundreds of metres underground.
Maps for Marina Bay, Jurong and Punggol were available first. More areas will be added over time.
Singapore laws limit land ownership to a maximum of 30m below the surface. Going way below that limit opens up vast new opportunities as new developments no longer need to track what lies above and be held back by private property ownership rights.
It gives land-scarce Singapore greater latitude to build all kinds of infrastructure - from rail networks to reservoirs and storage facilities.
FINDING SPACE FOR THE FUTURE
The idea to develop underground spaces was first raised in 2013 by then National Development Minister Khaw Boon Wan, who wrote in a blog post that Singapore could take a leaf from Canada and Japan's book, as their cities have extensive underground pedestrian passages, shopping malls and offices.
One of the "largest and most well-known" underground cities is Montreal's RESO, he wrote.
"It comprises 32km of tunnels covering about 12 sq km in downtown Montreal, linking a wide range of facilities such as offices, hotels, retail shops, cinemas, universities and train stations. It is easily accessible and half a million people use it every day."
Singapore, he suggested, could similarly maximise its land area by moving below ground transport hubs, pedestrian links, cycling lanes, utility plants, storage and research facilities, industrial buildings and shopping districts.
While many of these facilities remain on the wish list, Singapore already has extensive underpasses linking major shopping malls such as Ion Orchard, Tang Plaza, Wisma Atria and Ngee Ann City along Orchard Road.
In 2000, Singapore's first dedicated underground mall, Citylink Mall, opened. Spanning 60,000 sq ft (5.6 sq km), it allows people to shop while they walk from Raffles City to Suntec City Mall and Marina Square.
Why is Singapore looking underground? What other options does it have? In the past two centuries, the country has aggressively reclaimed swathes of land to meet the demands of its growing population.
While land reclamation has increased Singapore's land area from 58,150ha to 71,910ha, it can only go so far. The rising cost of imported sand, the deleterious impact on the ecosystem, shipping lanes as well as territorial limits impose restraints on the land reclamation option.
Other ways to maximise land space include packing more people and floor space into new developments, and rezoning old industrial areas and golf courses for homes and offices. But a city's livability must be considered, thus limiting the number of tall buildings that can be squeezed onto a plot of land.
These constraints support the move to dig deeper.
In announcing the URA's upcoming subterranean master plan last year, its chief planner Hwang Yu-Ning said: "The underground plan is part of our strategy to create spaces for the future and create capacity for growth." The plans also serve to keep developers informed.
At last month's Geospatial World Forum 2019 in Amsterdam, which The Straits Times attended, Singapore Land Authority's (SLA) chief executive Tan Boon Khai noted that as the country is "almost fully built up on land, we have to plan and build underground".
Even so, there are no plans to build underground homes. "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," he said.
LOCATING UNDERGROUND ASSETS
Whatever the grand vision, planners will need to deal first with a major challenge: find out what is hidden underground to avoid costly accidents, construction delays and disruptions to essential services and the broader economy.
Accidentally nicking buried water and sewerage pipes, power grids and telecommunications cables comes with varying costs. Research by Britain's University of Birmingham has estimated that it ranges from £300 (S$530) for damage done to a water or sewerage pipeline to £2,800 for fibre optic cables. Potential business losses arising from service disruptions are estimated to be 30 times that of each hit.
Singapore has suffered 26 cases of telecommunication cable cuts in the past three years that resulted in hours or days of Internet and telephony service outages. As connectivity becomes increasingly important to businesses and home users, the price incurred with each incident rises.
These cables are typically laid up to 3m below ground. The challenge lies in not just physically protecting them whenever the ground is dug up for new train lines, power cabling, water and sewerage works and the like but in locating and mapping them in the first place.
The process is complex and time-consuming because Internet service providers and telcos - Singtel, StarHub, M1, ViewQwest and MyRepublic - are unwilling to share the location of their assets, leading to a patchy knowledge of the island's buried critical infrastructure.
Outside the telco sector, the Energy Market Authority keeps track of where its power grids are laid, and national water agency PUB manages its own database of its water pipes.
There is no central agency from which earthwork contractors could get a consolidated, accurate view of all the underground assets on the island. This has meant that contractors have to repeatedly dig "trial holes" to check for buried utilities in many places, and determine whether diversion works are needed. This process is extremely unproductive and costly.
"The unknowns underground and the risk of hitting buried infrastructure have contributed to delays and inflated construction costs by up to 30 per cent," said Mr Geoff Zeiss, principal of the US-based consultancy firm Between the Poles.
ROLE MODEL IN THE NETHERLANDS
The unknowns include soil condition and the presence of aquifers and gas fields, all of which affect construction.
Soil condition, for instance, will determine the amount of reinforcement required for tunnels: Some stretches along the Thomson-East Coast MRT line near Katong Park, for instance, had to be strengthened as the area is made up primarily of soft marine clay, which has the consistency of peanut butter.
Capturing such information in a shared database is a step towards smarter underground city planning.
The Netherlands, which created a law to mandate the sharing of data on soil and groundwater from 2018, offers some valuable lessons.
The Dutch government also introduced an open database - called the Basis Registratie Ondergrond (BRO), or Key Registry for the Subsurface - accessible to all citizens. Any company that excavates or drills underground will have to share data with the BRO registry. Any user who finds inaccurate information is required to report it.
All the information will go towards the creation of an authoritative and comprehensive 3D underground map, the foundation for future underground development as well as ground-level construction planning. Having a rich and constantly updated database will help, for instance, engineers 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.
This kind of data-sharing looks like an uphill task in Singapore because of longstanding rivalry among companies and security concerns.
SLA's Mr Tan said: "One of the challenges is to get utilities companies and telcos to share data. They guard their data jealously."
But without knowing with certainty what is underground, it is hard for city planners to figure out whether there is scope to do more. As such, Singapore's progress has been limited.
One completed project is the Jurong Rock Caverns, a storage facility for liquid hydrocarbon 130m under Jurong Island. This has freed up more than 60ha, an area equal to 84 football fields, of development space on the island. The project is straightforward as Jurong Island consists mostly of reclaimed land, and it is managed by one agency - JTC Corporation.
More ambitious projects in the pipeline include new sewers 42m below the surface and reservoirs more than 150m below ground. The PUB's Deep Tunnel Sewerage System (Phase 2), to be completed in 2025, will traverse 100km across the western half of Singapore. The agency is also looking to store water in underground reservoirs at 150m and below to free up to 3,700ha or 5 per cent of Singapore's total land space.
The new projects need data, and mapping is a key first step.
On the plus side, new technologies that let underground assets be mapped with greater precision, with a margin of error of below 40cm and in 3D, make the job of avoiding earthwork accidents easier.
However, more needs to be done to improve the sharing of underground information and maps, with the Dutch model offering lessons for Singapore.
The subterranean project is a challenging one. If above-ground airports and ports can take decades to plan and develop, what more underground cities and amenities. But the rewards of more space and a more liveable city for all are immense too.
Greater efficiency in underground city planning will go a long way to advance Singapore's march towards this new frontier.