A grizzly bear exhibit at the Minnesota Zoo is temporarily closed after one of the three bears in the enclosure picked up a basketball-sized rock and shattered a barrier.

Startled visitors stood on the other side of the barrier as the bear repeatedly slammed the rock into a pane of glass at the zoo in Apple Valley on Monday morning.

The five-layer pane stayed in place 'like a windshield' and no one was hurt, zoo animal collections manager Tony Fisher told the Minneapolis Star Tribune.

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A grizzly bear exhibit at the Minnesota Zoo is temporarily closed after one of the three bears picked up a basketball-sized rock and shattered a barrier (pictured above the shattered pane following the incident)

Above Kenai the bear from the Minnesota Zoo who is thought to have slammed the rock against the glass. He is pictured here only minutes before the incident

The grizzlies were called back inside and the area will be closed until a temporary fix can be made ahead of the installment of a permanent, custom-made pane, Fisher said.

He isn't sure which of the three bears is responsible for busting one of pane's five layers, but he suspects it was Kenai - a nearly full-grown male who is 'usually the clown out there'.

'He didn't know what he was doing. He was just being a bear,' Fisher told the Minneapolis Star Tribune.

Robin Ficker, who was visiting from Maryland, said he was watching the bears as they wrestled with one another and looked on at the people shortly after the zoo opened at 9am, according to the Minneapolis Star Tribune.

Ficker said not long after, the grizzly bear 'picked up from the bottom of the pool a rock that had to weigh 50 pounds'.

'While many people were standing there, he slammed it against the glass several times,' Ficker continued.

Minnesota radio personality, Matt McNeil, was at the grizzly bear exhibit just before 10am with his kids and took pictures of the bear only minutes before the incident happened.

'For the record, I'm very glad my kids were not there when this bear did this. Minutes after we left,' McNeil tweeted.

Items in the exhibit that the bears are able to lift, including rocks, are supposed to be secured in place.

Following the incident, Fisher said the zoo will have to anchor down any items that can be lifted.

Grizzly bears have a life span of 25 years in the wild and usually weigh 800 pounds.

They can be particularly dangerous to humans if humans come in between them and their offspring or if they are surprised, according to the National Geographic.

Items in the exhibit that the bears can lift, including rocks, are supposed to be secured in place. Following the incident, Fisher said the zoo will have to anchor down items that can be lifted (above view of the grizzly bear exhibit at the zoo)