Instead, Nielsen and his colleagues used a creative method, based on the fallout from the nuclear bomb tests of the 1960s, to carbon-date the eyes of recently caught Greenland sharks. And they estimated that these creatures have a maximum lifespan of anywhere from 272 to 512 years, with a best guess of 392.

Even the lowest of those figures would make the Greenland shark the world’s longest-lived vertebrate, beating the previous champion—the bowhead whale, with an estimated lifespan of 211 years—by a significant margin. A 272-year-old shark would have been almost 170 when the Titanic hit the iceberg, and almost 90 when Charles Darwin set sail in the Beagle. It would have swum through centuries, cruising through icy waters while empires rose and fell around it.

Sadly, these ancient giants are sometimes snagged by fishing boats and research vessels. That’s how Nielsen first saw one. He was a graduate student on a research boat that accidentally hauled in a Greenland shark, while measuring cod stocks. “Everyone was up on the deck pushing it back into the ocean,” Nielsen recalls. “It was an amazing experience. When I got back, I thought: What was that animal? And hardly anything about its biology was known.”

A few months later, he was listening to a lecture on Greenland sharks. The speaker, John Steffensen, suggested that it might be possible to work out how old these animals get by studying the lenses of their eyes. These structures are made of proteins that are added in layers throughout the shark’s life. Peel away the layers and you can eventually find molecules that were laid down at the animal’s birth. The only problem with this idea was that Steffenson didn’t have enough lenses. “I raised my hand and said that I was on a ship that had caught and released a Greenland shark,” says Nielsen. “He said we should talk.”

Hunters in Greenland and Iceland used to catch some 50,000 Greenland sharks a year, and although fishing is uncommon, many are still caught as by-catch. Most are released without harm, but some suffer fatal injuries. And although the deaths of such old animals are tragic, Nielsen was determined that they shouldn’t be wasteful. So he and his team collected lenses from 28 of them.

For each lens, they measured the amounts of carbon-14—a mildly radioactive form of carbon. They then compared these measurements against a graph showing how carbon-14 levels have varied in the oceans over the past millennia. Such graphs are commonly used to carbon-date specimens on land, but the oceans complicate matters; there, carbon-14 levels haven’t changed much in the past centuries, and can vary from place to place depending on currents. To deal these uncertainties, Nielsen’s team had to learn to love the bomb.

Between 1955 and 1963, the world’s superpowers started testing their nuclear arsenal. The exploding bombs created huge amounts of carbon-14, doubling the usual levels in the atmosphere. As the carbon-14 spread around the world, it also worked its way into the food web. Animals, including marine ones, incorporated the stuff into whatever tissues or body parts they built at the time, effectively date-stamping themselves. Thanks to the brief era of nuclear testing, scientists have been able to accurately carbon-date everything from trees to elephant ivory to human brain cells. And now: Greenland shark eye lenses.