One of the most remarkable marine mammal fossil sites in the world was discovered by chance back in 2010, when construction workers began expanding the Pan-American Highway in the Atacama Desert region of northern Chile. After the workers reported finding bones buried in ancient sandstone, scientists were able to excavate more than 40 skeletons, including dozens of baleen whales, that they estimated had lain undetected for some 6 million to 9 million years. Now, a team of international scientists has published the most comprehensive review yet of the fossils, and shed some light on the mystery of how the massive graveyard was created.

Locals have long known that a certain region of Chile’s Atacama Desert contained the bones of ancient whales, and could even spot some of them sticking out of rocks. For this reason, they christened the site Cerro Ballena, or “Whale Hill.” But scientists had no idea such a remarkable marine mammal fossil site existed at Cerro Ballena until after an expansion of the Pan-American Highway began in 2010.

After road workers uncovered bones buried in ancient sandstone along the highway route, scientists were given only two weeks to complete field work before construction resumed. Nicholas Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History and the lead author of the new research on the Cerro Ballena site, first called in a team of technicians (nicknamed “laser cowboys”) to create a 3-D digital record of the arrangement of the skeletons, so that replicas could be printed later. The bones were then excavated and shipped to museums, where they would be examined by U.S., Chilean and Brazilian scientists.

According to their findings, published this week in a Royal Society journal, the Cerro Ballena site contained more than 40 skeletons, including dozens of baleen whales. Since whale skeletons are usually found one at a time, the discovery of so many together was significant in itself, and raised questions about how so many animals so high on the food chain were killed and preserved in the same location. The scientists immediately noticed that the skeletons were nearly all complete, and many had similar death poses (belly-up and facing the same direction), which suggested they died at sea or shortly after washing ashore. According to the research conducted by Pyenson and his team, the skeletons were strewn across a small area of land in four distinct layers, suggesting they were deposited there in four separate mass strandings, at different times over a period of more than 10,000 years during the late Miocene epoch, some 6 to 9 million years ago.

In addition to whale fossils, the Cerro Ballena site also yielded the remains of two seals, a now-extinct species of sperm whale, a toothed whale resembling a walrus and an aquatic sloth. As Pyenson told BBC News, “It’s amazing that in 240m of road-cut, we managed to sample all the superstars of the fossil marine-mammal world in South America in the Late Miocene. Just an incredibly dense accumulation of species.”

As to how the animals died, Pyenson and his colleagues believe the strandings were the result of algal blooms known as red tides. After consuming contaminated prey, or simply inhaling the algae itself, they believe, the whales and other animals were washed onto an estuary and eventually onto the flat sands where they were buried at Cerro Ballena. Once a tidal cove, the land containing the skeletons has since that time been lifted about 120 feet above sea level by the same tectonic forces that created the Andes Mountains.

Cerro Ballena is now regarded as one of the densest fossil sites in the world, and certainly one of the richest fossil collections of whales and other marine mammals. According to Pyenson, its archeological significance is comparable to sites such as the La Brea Tar Pits, which have yielded the remains of mammoths and other huge prehistoric beasts, and Dinosaur National Monument. He and other scientists working in the Atacama region suggest that there could still be more discoveries to come, in the form of hundreds of other fossil finds (including whales) that may remain hidden in the sand and rock lining the Pan-American Highway.