By Olivia B. Waxman
April 18, 2019

When the Passover holiday begins on Friday at sundown, Jewish people around the world will celebrate by retelling the biblical story of the Exodus from Egypt — including the 10 plagues that God inflicted on the Ancient Egyptians.

As the Passover story tells it, after Pharaoh refuses Moses’ entreaties to let the enslaved Israelites go free, God sends a series of ten plagues to pressure the Egyptian ruler. Each time, Pharaoh promises to free the Israelites, but reverses his decision when the plague is lifted — until the last one. The plagues are: water turning to blood, frogs, lice, flies, livestock pestilence, boils, hail, locusts, darkness and the killing of firstborn children.

The question of whether Bible stories can be linked to archaeological discoveries is one that has long fascinated scholars. The ten plagues are no exception, and over the years scientists have been curious about whether the story of the plagues may have been based on some event that can be proved to have happened. Here are three of the major theories to know.

Volcanic Eruption

This theory argues that the plagues were really the fallout of volcanic eruption on the island of Santorini in the south of Greece around 1620-1600 BCE. Microbiologist Siro Trevisanato, author of The Plagues of Egypt: Archaeology, History and Science Look at the Bible, argues that ancient Egyptian medical texts support this idea.

Winds would have carried the volcanic ash to Egypt at some point over the summer, and the toxic acids in the volcanic ash would have included the mineral cinnabar, which could have been capable of turning a river a blood-like red color, Trevisanato holds. The accumulated acidity in the water would have caused frogs to leap out and search for clean water. Insects would have burrowed eggs in the bodies of dead animals and human survivors, which generated larvae and then adult insects. Then, the volcanic ash in the atmosphere would have affected the weather, with acid rain landing on people’s skin, which in turn caused boils. The grass would have been contaminated, poisoning the animals that ate it. The humidity from the rain and the subsequent hail would have created optimal conditions for locusts to thrive. Volcanic eruptions could also explain the several days of darkness — which means nine plagues are accounted for.

Trevisanato also found an ancient Egyptian account of the children of aristocrats lying dead in public and archaeological data matching the account. He believes that, amid all this destruction, firstborn children could have been sacrificed out of desperation, in the hopes that such a meaningful sacrifice would lead their gods to stop punishing them.

Red Algae

This theory — put forth by scientists like John S. Marr, an epidemiologist who wrote a 1996 journal article featured in the New York Times — argues that red algae could have sucked oxygen out of Egypt’s waterways, killed the fish and turned the water red. Just as in the volcano theory, frogs then leapt out looking for food, and died. Without frogs to eat the insects, the pests proliferated and feasted on corpses, a feeding frenzy for flies and locusts. The paper argued that the lice could have been a type of insect called culicoides, which can carry two diseases that could explain the livestock deaths: African horse sickness and Bluetongue. The boils on humans could have been caused by glanders, an airborne bacterial disease spread by flies or tainted meat.

In this theory the darkness is coincidentally caused by a sandstorm. The darkness would have left the crops — well, whatever crops were left after the other problems — moldy, and the mold could have produced airborne toxins that might explain widespread childhood death.

A copy of a 15th century German manuscript depicting the plague of insects.
Historica Graphica Collection/Heritage Images—Getty Images

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Climate Change

This addendum to the algae theory points out that, for red algae to flourish in the first place, there needs to be slow, sludgy, warm water. In 2010, research on stalagmites —elongated mineral deposits that form out of calcium in precipitation — suggested that there had been a dry period towards the end of the rule of Pharaoh Ramses II. That change would have dried up the Nile and significantly slowed down the flow of water, according to paleoclimatologist Augusto Mangini. These conditions are ripe for the growth of the bacterium Oscillatoria rubescens, more colloquially, Burgundy Blood algae, according to biologist Stephan Pflugmacher.

But ask someone who’s celebrating Passover, and they’re likely to say that the question of whether scientists can prove the plagues really happened in ancient history is irrelevant to the holiday.

“To me it doesn’t matter whether scientists are able to find a historical basis for something that happened around 3,500 years ago. The central message is that God brought the plagues on Egypt in order to free the Israelite slaves,” says Jerusalem-based Rabbi Yonatan Neril. God was teaching the ancient Egyptians a lesson about justice, he says, and when they refused to do the right thing and free the Israelites, they suffered the consequences.

However, Neril — as the co-founder and executive director of the Interfaith Center for Sustainable Development — does say that science has a key role to play in how the story of the plagues is told and received today. That’s because he sees another lesson in it, one for the era of man-made climate change. The pollution of Ancient Egypt’s water echoes water-pollution and water-scarcity issues today. Frogs, with their very sensitive skin, are an indicator species, meaning that their suffering is seen by scientists as a marker of a larger imbalance in society around them. And to Neril, the parallels don’t stop there.

“The Egyptians were very happy to have a free source of labor in the form of Israelite slaves. When God said this needs to stop, they were reluctant to change,” he says. “Fossil fuels, in the past 150 years, have replaced slave labor as the key driver of human society. There’s a Pharaoh within us that wants to continue to do something that’s not right.”

Correction, April 18:

The original version of this story misstated Yonatan Neril’s affiliation with the Interfaith Center for Sustainable Development. He is the founder of the organization, not a co-founder.

Write to Olivia B. Waxman at olivia.waxman@time.com.

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