As part of the first full excavation of Egypt's ancient Dog Catacombs, scientists examine 2,500-year-old animal remains—a small sample of the roughly eight million animal mummies in these tunnels.

Likely supplied by ancient puppy mills, most of the mummies are dogs, and many were just hours old when taken for mummification, according to new research based on the summer 2010 excavation.

Snaking beneath the desert at the ancient royal burial ground of Saqqara (map), the Dog Catacombs were discovered more than a century ago. But only now is research shedding light on the massive number of mummies found in this complex of tunnels and chambers dedicated to Anubis (picture)—jackal-headed god of the afterlife.

Poorly mummified and piled high, the carcasses long ago deteriorated into indistinct heaps, experts say.

"It's not easy to identify individual mummies in the galleries or in the photographs," said Paul Nicholson of Cardiff University in the U.K. "We have piles of mummy remains just over a meter [three feet] high, on average, that just fill the side tunnels.

"Although the mummies are not well preserved or well decorated, unlike some museum specimens, they can still give us a great deal of scientific information," Nicholson added.

The mummies were stacked between about the late sixth century B.C. and the late first century B.C., Nicholson said.

(Related: "Dog Mummies Found in Ancient Peru Pet Cemetery.")

Pictures: Millions of Puppy Mummies in Egypt Labyrinth

Perhaps supplied by ancient puppy mills, millions of animal mummies—likely seen as conduits to a god—fill newly excavated tunnels.