SINGAPORE (AP) – Already one of the most densely populated countries in the world, tiny land scarce Singapore is projecting its population to swell by a third over the next two decades. To accommodate the influx, its planners envisage expanding upward, outward and downward.

The population target of 6.9 million people, an increase of 1.3 million from the present, is contentious in a country where rapid immigration has already strained services such as public transport and contributed to surging home prices and a widening wealth gap. It sparked a rare protest last week, with some 3,000 people gathering in a park that's the only approved area for demonstrations.

Singaporeans, whose forebears mostly hailed from southern China, fear their falling birth rates combined with the relentless immigration will reduce them to a minority in their own country. Adding a new dimension to their complaints is the idea that planners want underground living to leap off their drawing boards and become a solution to overcrowding.

State media is already championing the idea. In September, the Straits Times newspaper characterized underground living as the "next frontier'' for Singapore. It said Singaporeans may one day "live, work, and play below ground in vast, subterranean caverns that make today's underground malls look like home basements.'' The Building Construction Authority, which oversees a new agency responsible for surveying underground, said it could become reality by 2050.

The public's reaction has included derision and disbelief.

"Why pull me down,'' said Patricia Bian-Hing, a retired 87-year-old businesswoman. "The only time I will go underground peacefully to live will be in my coffin.''

But experts are calling for an open mind about the possibility.

"Singaporeans are dismissing this prospect because it is new, not because it is unworkable or implausible,'' said Jeffrey Chan, an assistant professor of architecture at the National University of Singapore.

"Astronauts who live in space stations, despite the abundance of direct sunlight have to live in shade most of the time, and they are only debilitated from the lack of gravity, not light,'' he said. "Hence, I think if there are any biologically-imposed constraints, psychologically or real, these biological constraints can be overcome through new habits or technologically.''

With about 675 square kilometers (261 square miles) of land, Singapore is only 3.5 times the size of Washington DC and has limited options for increasing its space. Land reclaimed from the sea already accounts for a fifth of its landmass and Singapore's appetite for imported sand for reclamation has caused tensions with neighboring countries concerned about coastal erosion. But its ruling People's Action Party, in power since 1959, sees a bigger population as crucial to its goal of transforming Singapore into what it calls a leading world city.

The government's new plans call for releasing land for housing and industry by closing golf courses and military training grounds and paving over some of the island's nature reserves.