In 79 A.D., Mount Vesuvius erupted, blanketing the nearby Roman towns of Pompeii and Herculaneum in hot ash and preserving the casualties in lifelike poses. And as awful as being smothered by ash may be, a new study suggests that suffocation wasn’t the cause of death for many victims.

Archaeologists have found that some people perished in a pyroclastic surge, a wave of superheated gas and hot ash that literally boiled their blood and caused their skulls to explode, reports Neel V. Patel at Popular Science.

The evidence comes from boat houses in Herculaneum, a seaside resort town for wealthy Romans about 11 miles from Pompeii. In the 1980s and 1990s, archaeologists began uncovering the remains of several hundred people who had huddled in the shelters at the water's edge to wait out the eruption. For hours the volcano, which had not erupted for hundreds of years, shot ash and chunks of pumice into the air, causing many people to evacuate or to seek shelter in solid structures. But it appears that a flow of superheated gas rolled down the mountainside at hundreds of miles per hour and blindsided the people in the waterfront chambers.

The new study, published in the journal PLoS One, presents more evidence that the boat house victims were killed by heat, not suffocating ash fall. George Dvorsky at Gizmodo reports that researchers examined 100 samples of bones and skulls using special types of spectrometry that can detect very low concentrations of minerals. The team looked at strange red and black residues found on the bones, determining that they had unusually high concentrations of iron. Those types of concentrations occur in two types of situations: when metal objects are subjected to high heat, and when blood is boiled away.

The skulls of the victims also showed signs they were subjected to high heat. In particular, many of the skull-caps showed signs that they had exploded outward and also had residue on them. It’s believed that the 400 to 900 degree heat boiled the fluid in the victims’ heads causing their skulls to explode and instantly turning their brains to lumps of ash.

Patel at Popular Science reports that, though the death is pretty gruesome, it was probably mercifully quick. Since the residents of Herculaneum were closer to the mountain than people in Pompeii, the heat was more intense, says Pier Paolo Petrone, lead author of the study from Federico II University Hospital in Naples, Italy. Previous studies show that people in Pompeii likely also died of “heat shock.” Because these victims were farther away, the heat was only 200 to 250 degrees, and they did not sustain the same types of injuries as those in Herculeneaum. Gizmodo’s Dvorsky reports that many of the ash-corpses in Pompeii are curled into what archaeologists call the “pugilist” position, likely because the heat caused their muscle fibers to contract. In Herculeneaum, however, the bodies seem more naturalistic, likely because the intense heat turned their muscle to ash before they had time to curl up.

Giuseppe Mastrolorenzo, a volcanologist at the National Institute of Geophysics and Volcanology in Rome who was not involved with the study, tells Popular Science’s Patel that the paper does not definitively show that the boat house victims died of heat. He points out that something else could have killed them, then their blood and brains boiled sometime after death. It’s even possible that the black and red residue was caused by metal on the victims bodies and more study is needed.

Whatever the case, the work highlights one of the overlooked dangers of volcanoes. While many people focus on the clouds of ash and slow-rolling lava, the real damage comes from the pyroclastic flows of gas and ash unleashed by an eruption. It’s estimated that Vesuvius’s 79 A.D. eruption was 100,000 times more powerful than the atomic bombs dropped on Japan at the end of World War II.

That’s pretty frightening if you consider that modern-day Naples, a city of 3 million people, lies about 8 miles from Vesuvius, which tends to erupt every 2,000 years or so. You do the math.