As Singapore developers start gearing up for a subterranean future, experts have warned of the pitfalls of going underground.
They say plans for a possible network of tunnels, malls and research labs could fall foul of the island's patchy soil formations and built-up landscape.
These factors could push up costs and make life difficult for planners, who would need to get even more businesses on board.
On the other hand, burrowing into the earth could provide valuable room to build in space-scarce Singapore."Our land boundary is finite," said Professor Yong Kwet Yew of the National University of Singapore's civil and environment engineering department.
"However, the only limit on underground space is the commercial viability of the project."
Earlier this month, National Development Minister Khaw Boon Wan said in a blog post that the Government was mulling over the possibility of an underground masterplan to make the city "even more exciting and liveable".
These developments could include malls, pedestrian links, cycling lanes and research facilities.
Yet, engineers and analysts say building them could be hard going in many areas due to the the varied nature of Singapore's rock and soil formations.
For example, Bukit Timah and Bukit Gombak have tough chunks of granite and norite, while stretches of West Coast Road sit on limestone deposits. These rocks can be harder than concrete, making excavating them a costly business.
In Jurong, the rocks and soil have been weakened by rain and high temperatures. Meanwhile, Kallang has loose sand, soft clay and silt, although these can be treated and strengthened.
Another hurdle is getting businesses, developers and the public to buy into the project.
This can be especially tricky in urban areas.
Malls in Orchard Road have made the news for saying "no" to basement walkways between buildings, despite a study showing that these could lead to higher profits in the long run.
"Once an area is built up, you would need to get more stakeholders on board if you want things to move forward," said Orchard Road Business Association executive director Steven Goh.
Then, there is the issue of what kinds of development will work best beneath the surface.
"Retail spaces, warehouses, industrial buildings and certain types of offices can be sited underground, where there are no windows and it's artificially air-conditioned," said DP Architects director Vikas Gore. "But putting homes in there would be problematic, as people might have difficulty adjusting to them."
With taking the subterranean route already an expensive option, factors such as unsuitable rock types could cause the price tag to rise. SLP International head of research Nicholas Mak said building one level underground costs about the same as building three on the surface.
Nevertheless, Singapore already has a number of subterranean developments such as the Jurong Rock Caverns, which store petrochemicals and oil.
There are also plans for an underground science city beneath Kent Ridge Park, with a street-type layout spanning 192,000 sq m of rentable space.
Prof Yong said more studies should be done to map out soil conditions. "Everything is a matter of cost," he added. "At the end of the day, it depends on whether the ground conditions are suitable for what you want to build."