California Zoo Gorilla Cracks Enclosure's Glass in Front of Horrified Visitors, Who Say It 'Felt Like an Earthquake'

In footage of the frightening moment, captured by Emily Houglin and shared with PEOPLE, Denny charges at the glass wall of his habitat, which he is sharing with another gorilla. The powerful primate makes contact with the glass wall with a heavy thud, which leaves the group near the wall gasping and yelping in fear. After the shrieks stop, the clip shows the substantial crack Denny left behind.