Ancient Egypt may be better known for its human mummies, but Egyptians also commonly mummified their animals as well. In addition to house pets buried alongside their owners, a large market existed for mummified animals—from birds to cats to crocodiles—intended as sacred offerings to Egypt’s many gods. So large, in fact, that an entire industry arose, which researchers believe produced more than 70 million animal mummies between 800 B.C. and 400 A.D. But a new study analyzing hundreds of these mummies has revealed that many of them contained only partial remains—while others contained no animal remains at all.

Ancient Egyptians created animal mummies for various reasons. Some were household pets buried alongside their deceased owners, or other animals that held special importance to the humans around them. Some mummified animals were intended as food offerings to humans in the afterlife. Many others were created to serve as sacred offerings to the gods, who in ancient Egypt often took animal form, including cats, cows, falcons, frog, baboons and vultures, among many others.

Animals mummified for this last purpose were available for purchase or barter at sacred sites. The people who bought them would often give them to a priest, who would then bury collections of the animals as a gift for the gods. This practice, similar to the act of lighting a votive candle at a church, was so widespread in ancient Egypt that animal mummification exploded into big business. Archaeologists have found 30 catacombs in Egypt, each one dedicated to a single animal and each packed floor to ceiling with mummies, for a total numbering in the millions.

In the largest study of its kind, researchers at the Manchester Museum and the University of Manchester have used X-rays and CT scans to examine more than 800 ancient Egyptian animal mummies, many of which are now housed in British museums. The animals examined ranged from birds to cats to crocodiles, with many others in between. While a third of the mummies contained the well-preserved remains of complete animals, researchers found only partial remains in another third of the mummies. Most shockingly, one third of the mummies have been empty of all bones or other animal remains, with the linen wrappings stuffed with items such as mud, sticks, eggshells and feathers.

As Dr. Lidija McKnight, an Egyptologist from the University of Manchester, told BBC News: “We always knew that not all animal mummies contained what we expected them to contain, but we found around a third don’t contain any animal material at all.” McKnight and her colleagues estimate that some 70 million of these mummies were produced over 1,200 years, from roughly 800 B.C. into the Roman period, which ended around 400 A.D. Animal mummification was an industry, they believe, with a special breeding program for all different species of animals, many of which were killed when they were still young and small. Eventually, despite the industrial scale of this operation, the researchers believe the high demand for the animal mummies may have outstripped supply.

So was this all a massive scam? The researchers don’t think so; they believe the people burying the mummies probably knew they were fakes, or at least contained only partial remains. In fact, many of the materials used (such as the eggshells and feathers) would have been considered just as important as the animals themselves. As McKnight explains: “They were special because they had been in close contact with the animals – even though they weren’t the animals themselves. So we don’t think it’s forgery or fakery.”

This fall, in conjunction with the research, the Manchester Museum will open an exhibit on animal mummies, in the hopes of illuminating this little-seen aspect of ancient Egyptian culture. For the past several years, as part of an experimental program, McKnight and her fellow researchers have also been creating animal mummies of their own. Animal lovers shouldn’t worry, though: The new mummies are mostly birds, all of which died of natural causes.