A grizzly bear at the Minnesota Zoo in Apple Valley picked up a basketball-sized rock with both front paws Monday morning and repeatedly slammed it into a pane of glass, shattering the barrier as startled patrons stood on the other side.

The five-layer pane stayed in place "like a windshield," and no creatures on either side of the multiple panes were harmed, said zoo animal collections manager Tony Fisher.

The rambunctious bear and two other grizzlies in the open-air exhibit were called back inside, and the area remains closed to the public until a temporary fix can be made. Later, a permanent pane will be custom-made and installed at a cost of tens of thousands of dollars, Fisher said.

Robin Ficker, a visitor from Maryland, said he was watching "the bears wrestling with each other and looking at the people" shortly after the zoo opened at 9 a.m. Then one of the animals "picked up from the bottom of the pool a rock that had to weigh 50 pounds. And while many people were standing there, he slammed it against the glass several times," Ficker said.

Fisher said he's not sure which of the three bears is responsible for breaking one of pane's five layers, but he suspects it was Kenai, a nearly full-grown male. Grizzlies typically top out at 800 to 900 pounds.

Kenai is "usually the clown out there, fooling around in the water," Fisher said. "He didn't know what he was doing. He was just being a bear."

Rocks and other items in the exhibit that bears are strong enough to lift are supposed to be secured in place, Fisher said. In the wake of this incident, he said, "we'll have to go back through the exhibit and anchor down anything they can lift."