37 years ago today: on December 3, 1976, during photography at Battersea Power Station for the Animals cover, Pink Floyd’s 40 foot inflatable pig “Algie” broke free from his tether and flew away. The pig passed through Heathrow air space, causing flight delays.

Pink Floyd had insisted that the cover image be “real” rather than a photo of a pig “stripped in” (the old school film-based equivalent of “photoshopping”) on top of a photo of the power station.

The photo shoot began a day earlier on December 2. A marksman had been on hand to shoot the pig out of the sky if it broke free. But the pig was not launched that day, and nobody told the marksman to return the next day. However, Storm Thorgerson and his crew of 11 photographers had the foresight to take pictures of the power station, sans pig, under that day’s dramatic cloudy sky.

On the second day, Algie was launched and the photographers were busily snapping away when a gust of wind broke the mooring cable and set him loose. The pig was out of sight within five minutes. Police tracked the pig to 30,000 feet before giving up. Eventually he landed that evening in a farmer’s field in Kent.

Roadies recovered Algie from the farm and patched him up, and he was floated again for a third day of photography. This time there was no incident, but it was a sunny day with a boring clear blue sky. Ultimately the decision was made by the band to strip in an image of the pig taken on the third day on top of an image of the power station taken on the first day with the much less boring cloudy sky. The final cover art ended up not being real after all, it could have been composed without floating the pig, and the Great Escape could have been avoided entirely. But nobody ever created a legend by not trying.