SINGAPORE — Singapore, with a little less land mass than New York City, is running out of room for its 5.4 million people.

The city-state has built upward — with apartment buildings reaching as high as 70 stories — reclaimed underused properties for housing and pushed out coastlines for more usable land.

But as one of the world’s most crowded cities, and with projections for 1.5 million more people in the next 15 years, Singapore’s options are as limited as its space.

So Singapore is considering a novel solution: building underground to create an extensive, interconnected city, with shopping malls, transportation hubs, public spaces, pedestrian links and even cycling lanes.