Chimpanzees used a branch brought down by Storm Erik to escape their enclosure at Belfast Zoo.
Zookeepers blamed the weekend's stormy weather for weakening trees inside the chimps' corral, as visitors on Saturday had a closer encounter than planned.
The intelligent primates turned a branch into a makeshift ladder and scaled high walls surrounding their pen in Northern Ireland, footage posted to social media showed.
Shocked visitors described seeing one of the mammals emerge from the bushes onto a path at the animal park north of the city.
One child watching the spectacle exclaimed: "Don't escape you bad little gorilla."
Other chimpanzees were filmed sitting atop the walls. The great escape ended when the animals returned to their quarters.
It was the second breach in recent weeks after an endangered red panda went missing when electric fences failed.
The animal was recaptured in the driveway of a nearby house.
Belfast City Council, which runs the zoo, said: "Zookeepers were present as the chimpanzee quickly returned from an adjacent wall to the rest of the group inside the enclosure."
Zookeeper Alyn Cairns said trees in their enclosure have been weakened by the storms, allowing the creatures to break them and use them as a ladder to escape.
He said: "They’re intelligent primates and know they’re not supposed to be out of their enclosure, so got back in themselves."
Elaine Monaghan's grandchildren were at the zoo at the time.
She said: "Why weren’t the enclosures checked? This could have ended very badly.
"This is how close it came to my grandchildren."
She said she was happy the incident had ended well for her family.
"Having four children with them they tried to act as calmly as possible so as not to frighten the kids," she said.
"Everyone has their own way of reacting. Had it been me I’d have run for the hills."
A Belfast Zoo spokesperson said "Zookeepers carry out a range of checks as part of their daily duties, including ensuring enclosures are free from objects which could pose a security risk to animals or visitors.
"In this case, management are assessing whether the tree branch used in the escape fell after morning checks had been completed, having been weakened during high winds on Friday evening and into Saturday.
"This is a highly unusual event, despite the natural curious nature of chimpanzees, and we would like to reassure the public that Belfast Zoo is fully committed to providing a safe and enjoyable experience for all our visitors.
“Additional security checks are also being carried out at other enclosures following this incident, to ensure the health and safety of both animals and visitors, which remains a key priority for our dedicated team."