The rain kicked up right when the gates opened Saturday afternoon for Rock the Garden concert outside Walker Art Center, putting the event's adventurous and experimental spirit to the test — which it passed with flying colors and twirling rain ponchos.

Instead of postponing the performers' appearances or making fans stand out in the rain, opening act Dan Deacon did what any madcap, fun-loving psychedelic dance DJ would do. He turned it into Rock the Garage instead.

Thanks to portable speakers and electronic equipment, Deacon was able to move the party into the Walker's parking garage on short notice, and he created an unforgettable opening to what is already one of the Twin Cities' most momentous concerts of the summer every year.

"This is like a sick, creepy rave down in the catacombs," yelled Deacon, as fans danced between parked cars and concrete pillars.

At the set's end, he organized the audience into a long human trestle with arms clasped overhead, leading the crowd out of the garage just in time to catch the second band, Duluth trio Low — which proceeded to play a single, 35-minute droning song, "Do You Know How to Waltz," as the entirety of its set.

Despite — or because of — the fact that it's not like all the other festivals in town, Rock the Garden has long been one of the hottest tickets each summer and a showpiece for both the Walker and the Current, which use it to drive up membership. Their members get most of the tickets, which once again sold out in a day this year (with a few hundred extra issued this past week).

"It's a great perk and incentive for becoming a member," said Jason Ewoldt, 27, of St. Paul, a new pledgee of the Current's nonprofit parent Minnesota Public Radio, who didn't let the rain spoil his first Rock the Garden. "That's the risk worth taking with outdoor concerts, which we don't have enough of."

Ewoldt was most excited about Saturday's headliner, Canadian synth-rock band Metric. The band topped out a five-act lineup that also included Los Angeles alt-rockers Silversun Pickups and former Minnesota music punk icon Bob Mould of Sugar and Husker Du fame.

Playing a near-opposite set to Low — he wham-bammed 15 blistering tunes in an hour — Mould used the occasion to celebrate his former home state's recent legalization of gay marriage. "It's good for the economy," he boasted and went on to end his set with a song that touted gay equality in the early-'90s, "If I Can't Change Your Mind."

With the long-awaited sun blinding half the crowd from the side of the stage, Silversun Pickups similarly blared its way through its set, flaring up with bombastic Smashing Pumpkins-style rockers such as "Mean Spirits" before finishing with its infectious and climactic radio hit "Lazy Eye."

With dusk setting in, Metric wheeled out an ambitious neon light show and singer Emily Haines played the cool dance-party ringleader to go along with the throbbing art-punk grooves of "Help, I'm Alive" and "Youth Without Youth." It felt more like an arena-rock show than an arty museum party.

Still, many people at the end of the night were still talking about the unexpected garage party that started off the concert.

"That was unlike nothing your Dad and I have ever seen before," Mary Stanek told her two grade-school sons, attending their first-ever concert.

Brandishing a bullhorn and a broad smile, the Current's morning-show host Steve Seel also called it the most "spontaneous thing to happen in Rock the Garden's history," which dates back to 1998. It was such a surprise, Seel had to go through the crowd and "play the pied piper" to alert arriving fans to the concert's unintentionally perfect opening act: The underground music bash had literally gone underground.

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