LIMESTONE – Phish fans have learned to expect the unexpected.

At 2 a.m. Sunday, roughly 11/2 hours after the band’s last performance of the night and many of the tent city residents had climbed back into their nylon caves, a purple glow filled the base’s old control tower.

While small groups stared up at the growing light show, it took the ominous moan of synthesizers for many to climb out of tents and amble over to the tower’s base.

Perched on a makeshift stage at tower’s top, the Burlington, Vt., quartet broke into an unannounced, hour-long jam session of ambient jazz-influenced drum and guitar music.

Mesmerizing improvisation, the music only supported the larger impact of the performance art as it unfolded. As the strobes fired off in a ring around the tower’s perimeter, swirling blue circles danced in burning pink fireworks shot off in a nearby field.

“This isn’t even registering in my tiny brain right now,” said 24-year-old Joel Shreve of Pittsford, N.Y., adding that he had no clue the performance would occur. “This is so outlandish. I can’t even imagine what I’m witnessing. I feel totally at peace.”

The kicker came when three dancers rappelled from the tower’s window and spun suspended 50 feet up in anti-gravity pirouettes in the glow of the red and green spotlights.

Described as being similar to the events made famous in Tom Wolfe’s 1967 book “The Electric Kool-Aid Acid Test,” the visual “adult playground” took the idea of a Phish performance to a new level for 24-year-old Shaun Long of Dillon, Colo.

“This is epic s-,” Long said. “Phish has always been groundbreaking, but this is unbelievable. Perhaps we are witness to the future of what jam bands may be doing.

“The circus has come to town,” Long said, playing off the title of a Phish tune.

The surprise show marked a good starting point for Phish first-timer Robin Elson, 17, of Northampton, Mass.

“With my airstrip for a bed, I’m into it, out of it and all over it,” Elson said. “This is just them. It’s who they are.”