Updated, 9:38 a.m.

Good morning on this gloomy Friday.

In our cramped city of more than eight million people, there are still spaces and places that are blocked off, hidden behind walls, or resting below our feet.

Take, for example, a century-old bowling alley in the basement of the Frick Collection, on the Upper East Side.

Or the apartments at New York Public Library branches across the city, which were used to house caretakers and their families. (Thirteen of the apartments remain.)

Two Brooklynites, Sandra Williams and Frank Milliron, even stumbled on a secret room during a remodeling of their Victorian home in Ditmas Park.