First posted on October 11, 2013. Updated with this year’s dates.

On a Sunday in October a few years ago, I met a friend in SoHo to go to mass the Basilica of St. Patrick’s Old Cathedral. It’s a beautiful church and was the first cathedral church in the Catholic Diocese of New York. It was a gorgeous fall day and we’d heard there was some chocolate festival going on (read: free samples at all the local chocolate shops) so we decided to wander the neighborhood and check out a few.

We were meandering up Second Ave when we noticed a woman standing next to a table beside an open gate. Beyond the gate was a grassy courtyard. We stopped to ask what was going on and were handed a brochure for Open House New York, a weekend in October when interesting spots around the city that are normally closed are open to the public. The gate we were standing outside led to the Marble Cemetery.

The Marble Cemetery is apparently New York City’s oldest non-sectarian public cemetery. The 156 vaults under the ground hold all of the 2,000+ bodies, and there are plaques along the walls of the cemetery that record the names of the original owners. It’s a quiet, lovely little space hidden in the middle of a busy neighborhood, but it’s a bit creepy too, like most cemeteries.

Recently a friend recommended a book to me called Kiki Strike: Inside the Shadow City, by Kirsten Miller. It’s the first of three books featuring smart, snarky girl adventurers in NYC, and in it they discover a passage into a “shadow city”, a network of secret tunnels and rooms, beneath New York. The main entrance to the shadow city is through the Marble Cemetery. Next time I visit, I’m totally looking for it.

The Cemetery is open on certain Saturdays of the year, but it’s open both Saturday and Sunday this weekend (October 11 and 12) as part of the 12th Annual Open House New York. Some of OHNY’s events are registration-only, but many are not, and everything is free! If you’re in NYC this weekend and want to take advantage of the nice fall weather we’re supposed to get (on Sunday, at least!), check out their schedule of events, from tours of private residences to talks about architecture to family activities. And if you’re near the Marble Cemetery, make sure to walk up to Bond Street Chocolate or over to Vosges or MarieBelle—turns out most chocolate shops always have free samples!