But that may soon change, as the city-state prepares to unveil an Underground Master Plan in 2019.

With some 5.6 million people in an area three-fifths the size of New York City - and with the population estimated to grow to 6.9 million by 2030 - the island nation is fast running out of space.

Singapore has been reclaiming land for decades, but that is increasingly unsustainable due to rising sea levels and other impacts of climate change. So the city is going underground.

Singapore has already moved some infrastructure and utilities below ground, including train lines, retail, pedestrian walkways, a five-lane highway and air-conditioning cooling pipes. It also stores fuel and ammunition underground.

Now, the city wants to go further.

"Given Singapore's limited land, we need to make better use of our surface land and systematically consider how to tap our underground space for future needs," said Ler Seng Ann, a group director at the Urban Redevelopment Authority (URA).

"Currently, our focus is on using underground space for utility, transport, storage and industrial facilities to free up surface land for housing, offices, community uses and greenery, to enhance liveability," he said.

The Underground Master Plan will feature pilot areas, with ideas including data centres, utility plants, bus depots, a deep-tunnel sewerage system, warehousing and water reservoirs.

There are no plans to move homes or offices below ground.

Singapore joins only a handful of cities that are mapping their subterranean space, said Peter Stones, a senior engineer with the consultancy Arup, which did a study for URA comparing its use of underground space to other cities.

"Globally, underground spaces are still back of mind; it's a Wild West of development, with a first-come, first-served system," he said.

"Singapore wants to look at it holistically and have a master plan so it can plan and manage the use of its underground space, and avoid potential conflicts," he said.

From the catacombs of ancient Rome to step wells in medieval India and World War II bunkers, underground spaces have been used for a variety of reasons.

Helsinki and Montreal are considered leaders in underground urbanism, a movement focused on innovative ways to use underground spaces.

Arup's study found Singapore's underground rail density is slightly behind Tokyo's, and that it has the lowest density for underground pedestrian links.

The study also found Singapore trailed Hong Kong and Tokyo in underground road density as of 2014.

Of 180 kilometres of urban rail, nearly half are located below ground, as is about 10 per cent of Singapore's expressway network.

Besides the space crunch, the other driver for tapping underground space in Singapore is the weather, said Stones.

"You have rising heat and humidity, and increasingly heavy rainfall. People want to avoid that," he said.

"Plus, utility networks are subject to more wear and tear in these conditions, so placing them underground is a viable option," he told the Thomson Reuters Foundation.

Advanced technologies and careful design also mean planners are able to alleviate the monotony and claustrophobia of underground spaces, he said.

But since building underground is generally more expensive and complex than on the surface, Singapore will only do so "where it is meaningful and practical", URA's Ler said.

"In many cases, it does make sense to build underground, considering benefits such as land saving, improving the quality of the environment and better connectivity," he said.

"Our underground MRT (mass rapid transit) network and expressways in the city area are some examples where the benefits of going underground outweigh the higher construction cost and technical challenges."

Storing fuel in rock caverns freed up more than 60 hectares, or 84 football fields, of space. The city's reservoirs occupy about 5 per cent of Singapore's land, so moving them underground could create more space.

"The space is there, it's an asset, and we should use it. Having a detailed and rigorous plan is crucial," said Stones.

Australian Associated Press