If you’re impressed (or worried) by Seattle’s very rapidly expanding skyline, you might be interested to know that, although it’s ramped up lately, buildings of this scale have only been cropping up since the 1960s. The development boom led to 1963’s Monson Plan, which aimed to revitalize downtown Seattle following two decades of post-World War II urban decay.

The deal was that skyscrapers were allowed to be built as long they it provided some kind of public benefit. In these cases, a “public benefit” meant some kind of park or outdoor space that’s open to visitors, whether it be on the grounds surrounding the building or contained secretly within it.

Today there are at least 44 privately owned public open spaces—known as POPSes or POPOSes—sprinkled all around the city of Seattle. but mostly downtown. Office workers and tourists alike are invited to eat lunch, work, read a book, or otherwise just hang out during the building’s hours of operation. Several other cities worldwide have these too, including but not limited to New York, Auckland, San Francisco, Toronto, and Seoul.

The idea behind these spaces wasn’t, of course, for them to be secrets—they were intended to be used and enjoyed, of course. But largely, it wasn’t until 2009, when Seattle City Councilmember Nick Licata launched a campaign to get the word out, that they showed up again on folks’ radar. After dredging up old city documents, Licata managed to collate every POPS into a checklist and sent it to the City of Seattle’s planning department, along with the task of adding plaques to each that announce their availability to the public.

Although they’re located on private property, each POPS offers the same rights to its occupants as a public park would, such as free speech and freedom of assembly. (This issue was in the news back in 2011 surrounding the Occupy Wall Street protests, which largely took place in a POPS: New York’s gigantic Zuccotti Park.)

Here in Seattle, many of our POPSes are downtown, but they’ve been spreading across the map—some of the newer UW dorm buildings have private-public plazas and mid-block connectors, such as Elm Hall and Alder Hall on Campus Parkway, as well as a teeny pocket park at Epiphany School in Madrona.

All this said, some POPS are definitely sexier than others. Some of the more quotidian ones that you might know already are the “hillside terrace” with the concrete steps along University on the south side of the Seattle Art Museum (where the Hammering Man is), the dinky little nugget of seating outside of—but not exclusive to!—the Starbucks just across from the Westin Hotel on Fifth and Stewart, or the gum-stained plaza above the International District transit stop at Fifth Avenue South and Jackson Street. But other POPSes are positively thrilling to discover, like finding a $20 bill on the sidewalk. Here are a few of our favorites.