A grizzly bear at the Minnesota Zoo had enough of life in captivity.

Zoo officials say the bear picked up a 50-pound, "basketball-sized" rock and repeatedly slammed it against the glass barrier of his enclosure until one of the five layers of glass shattered.

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"At no time was there any danger of the bears getting out since there were still four layers of glass, but we're not going to trust it," zoo director of animal collections Tony Fisher tells ABC News

According to eyewitness Robin Ficker,  he was watching "the bears wrestling with each other and looking at the people" when 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 tells the Star Tribune.

The enclosure, which houses three grizzlies from Alaska, is temporarily closed until the glass can be replaced, which could take several weeks.

Zoo staff isn't sure which bear broke the glass but they suspect it was Kenai, who they describe having a 'playful, clownish personality' reports Gawker. They plan to anchor down the rocks and any other loose objects the bears could lift.

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