Singapore's surface-level space crunch has got the Government looking underground instead. This week, national water agency PUB announced that it will look at the feasibility of underground reservoirs, with the study expected to be completed by the end of 2017.

The underground drainage and reservoir system is likely to include tunnels to channel storm water, caverns for water storage, and a pumped storage hydropower system which harnesses the power of flowing water to generate electricity.

Here are other recently completed or planned subterranean projects.

Jurong Rock Caverns

Officially opened last September, this massive underground storage facility for petroleum products is Singapore's deepest underground project, sited 150m below Jurong Island - four times further underground than Singapore's deepest MRT station.

Comprising five nine-storey-high caverns, it can store up to 1.47 million cubic metres of liquid oil products such as crude oil and condensate, enough to fill 600 Olympic-sized swimming pools.

Such oil products are usually stored in large tanks above ground. The caverns thus free up 60ha of land, or the equivalent of 70 football fields.

The $950 million project, developed by JTC Corporation, took six years of planning followed by eight years of construction.

At the official opening, Prime Minister Lee Hsien Loong said it was "born out of this same spirit of innovation and determination" behind Jurong Island, another ambitious project that involved amalgamating seven southern islands.

A commercial facility, it has already leased two of the five caverns to Jurong Aromatics Corporation, to store feedstock for its upcoming aromatics plant on Jurong Island. The other three caverns are expected to be operational by 2016.

Underground Ammunition Facility (UAF)

Also built in rock caverns is Singapore's first underground ammunition storage, opened in 2008 by the Singapore Armed Forces.

The Underground Ammunition Facility was built under the old Mandai Quarry and took 10 years to complete.

Developed and managed by the Defence Science and Technology Agency, it is said to be the world's most modern underground ammunition depot.

About 300ha of land - the equivalent of 400 football fields - was freed up by storing the ammunition in the UAF instead of conventional above-ground depots.

Land is not the only thing which it saves. The use of automation and technology means that it needs 20 per cent less manpower to operate compared to a traditional depot, and insulation by the granite caverns means that it needs half as much energy for cooling.

Its depth has never been disclosed.

Underground goods movement system

Instead of being transported by road, goods could someday make the journey from the port to industrial estates by a series of underground conveyer belts or self-driving cars.

In May this year, JTC Corporation called a tender to study the feasibility of an underground goods movement system.

The tender asks for at least three tunnel alignments to link the future container port in Tuas to the industrial estates at Tanjong Kling, Jurong West and Gali Batu.

The Business Times reported that experts believe the underground goods movement system will be automated, with conveyor belts and self-driving cars being possible options.

Underground city

Cycling lanes, shopping areas and public spaces could go underground too, as the Government prepares to layout an underground master plan.

The idea of a plan for Singapore's subterranean development was raised by National Development Minister Khaw Boon Wan in a blog post in September 2013.

He noted that Singapore, like many cities, has placed noisy and polluting land uses such as utilities and transport underground.

But some cities have gone further, he added, with underground shopping malls and offices in Canada and Japan, the underground 'city' of RÉSO in Montreal which includes hotels and universities, and subterranean sports complexes and concert halls in Scandinavia.

"Taking reference from other cities, there are possibilities of creating underground transport hubs, pedestrian links, cycling lanes, utility plants, storage and research facilities, industrial uses, shopping areas and other public spaces here," he said.

To pave the way for the development of an underground master plan, two Bills were passed in Parliament earlier this year. They clarify ownership rights to underground and above-ground space, and allow for Government acquisition of such space.