The scent of death revealed the location of the grisly, cannibalistic end to a hopeful trip to California nearly 175 years ago. This is thanks to cadaver-sniffing dogs who seem to have proven the ability to detect death thousands of years after some burials at historic sites. The Donner Party was a group of pioneers that attempted to migrate from Illinois to California in search of opportunity via a wagon train that partly followed the Oregon Trail starting in the spring of 1846. Tragically, the expedition became “the worst disaster of the overland migration to California,” according to Britannica. A combination of inexperience, bad choices and bad luck all contributed to disaster for the families that initially began the wagon train. The party didn’t manage to complete their trip by the time winter struck, and became stuck in deep snow in the Sierra Nevada mountains, where they built makeshift camps for survival. With the help of cadaver dogs, researchers are just now honing in on some of the remains of the deceased in this location — and at other sites around the world. But how do these dogs do it? Sniffing Out the Donner Party Mystery Dozens of the nearly 90 people that started the Donner Party expedition died, with the final survivor leaving the makeshift camp in April 1847. At least some of the survivors had resorted to cannibalism, eating the remains of those who died over the winter. The specific location where many of the Donner Party met their end still isn’t completely clear. “This was the site called the Camp of Death,” says John Grebenkemper, a dog handler at the Institute for Canine Forensics (ICF). The nonprofit was established 25 years ago in San Francisco to help detect ancient burials. In recent years, Grebenkemper’s dog Kayle, specially trained to sniff out cadavers, may have identified several of the places where people in the Donner Party died based on whiffs of decay that have lingered for nearly two centuries. Dogs Digging for History Since he first became involved with ICF in 2007, Grebenkemper has worked in everything from tracking down unmarked Indigenous American graves to searching for the crash site of the aviation hero Amelia Earhart on an uninhabited island in the Pacific Ocean. But others have used cadaver dogs to track down truly ancient remains in Europe, sometimes dating back thousands of years. Read More: Amelia Earhart’s Final Resting Place “The dogs have a wide range of application,” Grebenkemper says. A Special Kind of Dog Most dogs have a great sense of smell, but not every pooch is suitable for the type of training to detect ancient burials. The right personality usually requires a level of obsession that would be a fault in most pets. “You want dogs that have a strong drive,” Grebenkemper says. When trainers look at a litter of puppies, they will select the ones that want toys more than the others for this kind of job. “Of course, a dog works for reward.” It takes a couple of years to train a cadaver dog. Grebenkemper begins their training on human bones. “I have some very old bones that came from archaeologists in Europe,” he says. Other bones can be bought commercially. Usually, the dogs learn to find the smell of the bones pretty quickly. At that point, trainers take them to old cemeteries to transfer the scent. Eventually, they stop giving them rewards every time, or the dog may mis-learn to give false positives — detecting human remains when there are none. “They eventually get really into the game, and they just want to do it,” Grebenkemper says. “They don’t need a reward.” Read More: Let’s Journey Through the Mind of a Dog How it Works Researchers aren’t sure what exactly the dogs smell. But at the base level, the olfactory sense is about chemical detection. When creatures die, certain microbes that were always inside them in smaller numbers begin to increase, hastening the speed of decomposition. As they eat flesh, the microbes release chemical compounds that dogs can smell. A certain mixture of these compounds is probably unique to humans, so that dogs don’t mistake people for dead animals, Grebenkemper says. Dogs will sniff out these remains, alerting to their owners often by sitting down, nose right above the area with the highest concentration of smell. But that doesn’t always mean that the pooches location is spot-on. Depending on what sits between the surface and the remains, the decomposing compounds may not waft up in a straight line. Remains might be buried a few meters away from where the dog alerts. Archaeologists can also confirm where to dig using ground penetrating radar. These devices can draw a more complete picture of what we can’t see beneath us, but they often work a lot slower than our four-legged friends. In general, Grebenkemper says, dogs are best at finding remains that are within a couple hundred years old. When Nearly Nothing Remains Though that doesn’t mean cadaver dogs can’t find older stuff. In fact, dogs have successfully detected bones dating back millennia. In Croatia, Belgian Malinois and German shepherds typically used for criminal investigations detected burial chests containing bones and artifacts around a prehistoric hilltop fort called Drvišica. The researchers first checked if it would work, seeing if the dogs would find graves that had been previously discovered using other methods. When this worked, they let the dogs search more widely, and discovered a number of other bones dating back nearly 3,000 years. In the U.S., ICF has detected even older Indigenous American remains — some dating back 9,000 years. Perhaps even more surprisingly, dogs can detect remains that are no longer there at all. In the 19th century, some Chinese immigrants buried their dead only temporarily before exhuming them and shipping them back across the sea to their home areas. The traces of human decomposition left in the soil was apparently enough for dogs to detect, though most of the body was removed, Grebenkemper says. It’s even possible that dogs can detect cremated remains, despite the fact that burning destroys much of the chemical compounds produced by decomposition. Dogs from ICF have helped locate the remains of people burned in wildfires, and Grebenkemper is working on blind tests to further test their ability to find these types of remains. Preserving Undisturbed Remains The possibility of past burials has often brought conflict between developers and Indigenous communities, especially in cases where there may be old bones in the area. “They don’t want their ancestors disturbed,” Grebenkemper says. But where it’s not taboo, dogs can provide a non-invasive method to detect past remains without ever touching a shovel. Grebenkemper has worked in several of these cases, including a project with the Muwekma Ohlone Tribe near San Francisco. Read More: Graveyard DNA Reveals 2,000 Years of Tribal Presence in California Dogs can find lost burials important for living descendants. They can also help tribes plan future development by marking off potential burial sites. Grebenkemper sees a positive future for the use of dogs in archaeology. Since ICF started more than two decades ago, handlers have made big improvements on training dogs. Now other groups around the world are training cadaver dogs. “I suspect the technique will continue to get developed and the dogs will get better at it,” Grebenkemper says.