Whales occupy an outsized place in our imagination. They are mammals, like us, but grand and mysterious, from their enormous size to their social lives to the great depths they swim in. For all our fascination with whales, there’s also a surprising amount we don’t know about them—including basic information like how old individual whales are, or how long they’re capable of living. Marine researchers have long sought an accurate technique for “aging” whales, because knowing how old whales are is key to answering many other questions about their lives. But such a technique has been elusive—until now.