There's no map of them, but they're showing up nearly everywhere, in parks and on sidewalks, bearing the hashtag #SwingBombSF.
"We saw this -- this swing just hanging from a tree. It was very random," said Diana Apanovich, visiting with her two daughters from San Diego.
The kids, 6 and 11, played gleefully on the swing, though they admit it's a bit confusing.
RELATED: San Francisco Civic Center transformed into yarn zoo
"I thought it was a little odd," said 11-year-old Alyssa.
"Did someone actually put this up?" asked 6-year-old Ava. "How'd they even tie up there?"
"How" isn't so much the question as "who." The only clue is the hashtag, and the SwingBombSF Instagram account that goes with it -- showing some 50 swings hanging from trees across the city. We recognized a few spots and went to check them out.
"I was really excited when i saw it, and i sat down and swung," said Nick Chang of the swing he found on a shaded street corner.
Chang and his friends appear to be all grown up, but they say they never outgrew their swing sets.
RELATED: Whimsical, tiny fairy doors delight Alameda residents
"San Francisco at least used to be a place where, you know, inner child wanted to come out, and that's why everyone's here: we're all grown children," said one of them.
Speaking anonymously by phone, a SwingBombSF team member said that's who these swings are for.
"Swings just really capture that youthful spirit, and they just bring joy to people -- regardless of age," the person said.
The SwingBomb masterminds told us they wanted to make San Francisco a real life playground for adults. Unfortunately for them, some of the adults who run the city decided that somebody had to be the grown-up.
"Is that branch strong enough?" asked Recreation and Parks Department operations manager Dennis Kern. "They did not come to us for us to take a look at what they wanted to do, so that we can ascertain that this is safe."
Kern said without a permit on file, they have no choice but to take the swings down -- once they find them. The absence of any map listing the swings' location was a very deliberate design choice.
"To create that element of surprise and joy and things that happen unexpectedly," the spokesperson told us.
As for the sidewalk swings -- the ones that aren't in any park -- SF Public Works plans to remove those too, and gave a similar explanation. Until they do, they may continue to spark conversation about the spirit of San Francisco.
"Says that we got a lot of swingers -- got a lot of people that get really high," Chang said as he swung to and fro. "I think this kind of sums us up. You know? In one beautiful metaphor."