When physics professor Joseph Dwyer heard that 323 reindeer, ranging across a plateau in southern Norway last Friday, had been killed instantly by lightning—their bodies splayed out in clusters on the ground like groves of felled trees—he was stunned.

“It’s not uncommon for cattle or horses to be killed in large numbers [by lightning], but I’ve never seen anything close to that,” said Dwyer, who teaches at the University of New Hampshire and is one of the world’s leading lightning experts.

An employee with the Norwegian Environment Agency discovered the carcasses after a vicious thunderstorm pummeled the Hardangervidda plateau. Seventy calves were included among the dead. In photos released by the agency, the animals are visible in piles up to 100 yards apart. “They were standing on a hill, moving up that hill,” Olav Strand, a senior researcher at the Norwegian Institute for Nature Research, told the New York Times. “They seem to have fallen dead on the ground, exactly where they stood.”

With no comparable mass-death-by-lightning-strike on record, the question is how so many animals could have been killed at once.

With no comparable mass-death-by-lightning-strike on record, the question is how so many animals could have been killed at once. Dwyer said in a phone interview Monday that it is possible the animals were dropped by a single ground strike, though it's impossible to know for sure. “It depends upon how much current was flowing during the strike,” he said.

The strike’s deadly range—whether it could have killed animals standing 100 yards apart—also depends on how conductive the soil was. The more conductive the soil, the quicker the current disperses when it hits the ground, and the less dangerous for animals or people standing on the ground. A less conductive soil, which is typical in drier areas like southern Norway, allows large voltages to build up across the ground, which are then left searching for somewhere to go. That, in turn, can lead to “step voltages,” which involve an electric current traveling overland and impacting any person or object connected to the ground. It's common for lightning-strike victims to be injured or killed by step voltages. Usually the current enters a body through one leg and exits through the other—well below a person’s heart and lungs. But for a four-legged animal whose heart is located between its front and hind legs, the voltage often passes through its heart before exiting.*

“So an animal like a reindeer could be more easily killed,” said Dwyer, who believes step voltages were the likely mechanism for last week’s event. “If you had the right conductivity and a very big lightning flash, it seems plausible that you could get a large number of animals killed like this, assuming they were all huddled close together.”

In addition, the fact that reindeer have no safe place to retreat during an electrical storm can turn an entire herd into a target, Dwyer said. “This event really shows how dangerous lightning is. People have a choice, but reindeer have to stay outside, and there’s really no safe place outside during a thunderstorm.”