As the Pan-American Highway snakes its way through Chile, it passes through a place called Cerro Ballena. The site is in the middle of the driest desert in the world, and just an hour away from the mine where 33 unfortunate miners were trapped a few years back. That particular stretch of road is vital for ferrying the mining equipment and extracted minerals that are fuelling Chile’s economic surge. For that reason, a construction company was tasked with expanding the highway from two lanes to four.

But at Cerro Ballena, they unearthed something odd: the skeleton of a whale.

And then another. And another.

Dozens of them; different species; adults and juveniles. The construction crews had unearthed an entire whale graveyard.

“I wouldn’t wish a whale skeleton on anyone,” says Nick Pyenson, an expert on prehistoric marine mammals from the Smithsonian Institution. “You’re going to have to dig a lot.”

View Images The dig site at Cerro Ballena. Credit: Nick Pyenson

Pyenson first saw the graveyard in 2011 at the urging of his Chilean colleague Mario Suarez. He was amazed. Chile is famously rich in marine fossils anyway. The local tectonic plates push what was once ancient seafloor to more accessible heights and since the area is so dry, there is little vegetation and lots of erosion. But even in this place, which offers up its treasures with unusual abandon, the graveyard was something else.

But Pyenson was also troubled. The construction crews were treating the fossils with great care, jacketing the bones in plaster and transporting them to museums. But through their good work, they were also divorcing the bones from their surroundings. Without knowing where they were buried or the properties of the surrounding Earth, it would be impossible for scientists date them. And they’d never be able to answer the obvious and burning question: why had all these animals died in this same place?

Context was everything, and context was rapidly vanishing. The crews would finish their work by the end of 2011, and Pyenson knew that time was running out. “I remember literally looking at skeleton after skeleton and thinking, ‘Okay, what are we going to do? This is not what I signed up for.’”

Enter Adam Metallo and Vince Rossi. Pyenson calls them the laser cowboys. They are part of the Smithsonian’s 3-D digitisation team, and they flew over to Cerro Ballena with a smorgasbord of lasers and other scanning equipment. They worked continuously for several days, creating virtual models of the entire site and all its fossils, at extraordinarily high resolution. The bones have now all been removed, but their resting place lives on in a digital guise.

View Images The laser jockeys at work. Credit: Nick Pyenson.

So far, the team have documented 40 separate skeletons, which are all between 6 and 9 million years old. Thirty-one of these were probably from the same species of rorqual whale—the family of giants that includes blues, fins and humpbacks.

There were other animals too, including a penguin, an extinct type of sperm whale, and two seals (one of which is new to science). There was a walrus-whale—a bizarre prehistoric dolphin with a tusked walrus-esque face. There was even an aquatic sloth.

All of these specimens were found in an area of roadcut just 240 metres long and 20 metres wide. Pyenson estimates that there are hundreds of skeletons still buried in nearby areas that the construction teams didn’t touch. “What did they obliterate when they built the first two lanes of highway?” he wonders. “They must have dug up bone after bone.”

Most of the rorqual skeletons were complete, well-preserved, and belly-up. There was even a group of two adults and a youngster, log-jammed together and beautifully preserved. All of this suggests that they died at sea and were washed onto tidal flats, where they were quickly buried. And since the fossils were found in four separate layers, these mass strandings happened repeatedly in this same place.

View Images La Familia–two adult whales and a calf. Credit: Nick Pyenson

Why?

What killed such a diverse group of animals, time and again? “Every explanation must work across all the taxa, and must also satisfy what you see about the bones and their arrangement,” says Pyenson. “And it has to explain the pattern four times.”

It probably wasn’t a virus, of the kind that’s killing off dolphins in the US. Such epidemics are usually species-specific, and unlikely to kill many different species including both mammals and birds. It wasn’t a tsunami either. A large wave wouldn’t have selectively killed larger animals and, besides, the surrounding rocks show that the area was mostly calm and stable.

Pyenson says there is only one cause that fits all the available evidence: these animals were poisoned by algae.

Algae thrive when coastal waters receive an unusual influx of nutrients, and some species release deadly toxins into the water. These ‘harmful algal blooms’ or ‘red tides’ are potent enough to kill whales and dolphins, which choke to death on the toxins.

At the end of 1987, fourteen humpback whales were stranded along the coastline near Cape Cod, USA, over a span of five weeks. Like the Cerro Ballena fossils, the strandees included males, females and calves. When scientists cut them open, they found plenty of Atlantic mackerel in their stomachs. And the mackerel, in turn, were loaded with algal toxins.

All the signs at Cerro Ballena suggest that those animals died in the same way. They showed no signs of injuries or bite marks. They ate a wide variety of diets but they were all at the top of their food webs, and such animals are more prone to algal poisoning.

There are also orange blotches on the bones, which Pyenson thinks are the remains of algal mats. On some specimens, he even found thousands of tiny spheres, which are exactly the right size to be dinoflagellates—the creatures behind harmful algal blooms. “It’s the closest we get to a smoking gun,” says Pyenson.

His theory is that iron minerals from the Andes in the east periodically washed into the ocean. This fertiliser was concentrated in the surface waters by upwelling currents, transforming the local algae into toxic blooms. Their poisons killed off local animals, which floated towards the coast and were stranded on the tidal flats.

This clearly happened many times in the past. Today, small whales and dolphins are often stranded en masse, but aside from the Cape Cod humpbacks, the large rorquals generally aren’t. Pyenson thinks that’s because so many of them have already died at the harpoons of whalers. Whale graveyards like Cerro Ballena may be a thing of the deep past, because we have turned the entire ocean into a whale graveyard.