Ball Square has a building labeled USA storage. It has 4 sides, each with some doors. Some of them were open over the weekend for various art thingies.
After the show What Cheer played by one of the sides, drawing people in. Then a lot of mist came out of the roof of the building. And then a guy on stilts in a patriotic outfit started walking around.
Then after all of the lights at Ball Square went out and an announcement went out over the PA that the power was out and therefore the factories would no longer be able to make their product, which is the ultimate product. Then for some reason I didn't quite get, it suddenly worked and a box was handed to the stilt guy who put it on a ramp which carried it up to the roof of the building and dropped it over. Sometime during all of this, music started playing over the PA and after a few minutes it became apparent that it was Phish. Doors opened so we could see inside, but all we could see was silhouettes due to screens blocking our view as the band member you could see from your side played; each one had their own house, so to speak.
The jam went on for an hour or so, started out very dark and mainly effects based, but sometimes Floyd-esque. There's an amazing 4-5 minute section about a half hour in that really does sound like Pink Floyd. It then went into another sound effects jam before they started playing the most bizarre version of Sleeping Monkey ever.
There was a lot of confusion going around with people thinking that they chose the wrong spot to stand in - no one could move to a different side since due to the location of the secret set, What Cheer attracting people, the timing of the set (not long after the show) and the rumor getting out, it was easily the best attended secret set (note that the Lemonwheel 4th set was not secret. It was announced from the stage) - because their view was blocked, not knowing that that was true for everyone. Moreover the speakers were surrounding the building in a circle, pointing in, making it sound like the music was coming from behind the crowd instead of in front of them, a very surreal effect.
It was very weird and very cool, easily the highlight of the weekend so far.