This polar bear at the Rotterdam Zoo in The Netherlands apparently got tired of being in the tank. As the video shows, the polar bear tried several times to break the glass with a rock.
Near the end, the polar bear successfully cracked the glass and startled the two onlookers who quickly took off running.
Fortunately, the enclosure held up. Vicks, the polar bear, is being kept in a temporary enclosure while this one is repaired.