When it comes to New York City real estate, most residents wish they had more space: to open up a wall and discover a whole extra room would be nothing short of a dream come true. But in a city with 27,000 people per square mile — the highest population density of any major US city — what you see is what you get, and usually what you pay for too.

But the idea of hidden rooms and passageways, even in New York City, is more than just the subject of millennial imaginations. A recent New York Times article tackled just that: hidden spaces, both new and old.

The older hidden spaces, surveyed by the Times, served practical purposes. In one Brooklyn church, for example, a stairway leads to a secret basement the size of a city block, once part of the Underground Railroad. Inside the Brooklyn Bridge, there’s a hidden Cold War bunker. Though you might enjoy the idea of secret passages for the novelty, many people have relied on them historically to stay safe.

That’s not to say secret rooms can only be bunkers. One couple in Brooklyn, for example, discovered a crawl space large enough to transform into a quirky playroom for their young daughter. Others pay contractors like Creative Home Engineering as much as $25,000 to build secret rooms into their homes for aesthetic or personal reasons.

In her three-bedroom apartment on 15th Street and Fifth Avenue, finance worker Sara Nainzadeh had a secret entrance built, opening up to a private office by a tug on a Shel Silverstein book. Asides from storing her safe, the room acts as a retreat from the world.

A couple in TriBeCa, the Watsons, have a secret room built entirely for books: a secret library that doubles as a guest room. Their Duplex on Warren Street is selling for $19.5 million.

Still, others have them implemented as a sort of long-term investment in an uncertain future. With the growing state of technological and government surveillance, many think a private escape is more than worthwhile. If there were needs in the past for such safe spaces, who is to say, 30 years down the line, they won’t be useful again?

Hopefully most of us will never need secret rooms for life or death reasons. But if such spaces do come back in style, why not run with it? A little intrigue in real estate is never a bad thing.