Oakland Zoo spokeswoman Erin Harrison said in a statement Tuesday that a child hit a rock against the glass several times, "shattering the laminate layer."
She says the bulletproof-strength glass at the enclosure where four grizzly bears live is made of six separate panes, each an inch (2.5 centimeters) thick.
She says another zoo visitor reported the child to staff, but they couldn't find the family.
Harrison says a replacement glass pane will cost $67,000 and require specialty equipment to install next month.
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