Why the Search for Dark Matter Depends on Ancient Shipwrecks

In 2017, Chamkaur Ghag, a physicist at University College London, got an email from a colleague in Spain with a tempting offer. The year before, an emeritus professor at Princeton  University, Frank Calaprice, had learned of old Spanish ships that had sunk off the New Jersey coast 400 or 500 years ago, while carrying a cargo of lead. Calaprice obtained a few samples of this lead and sent it off to Spain, where a lab buried within the Pyrenees tested its radioactivity. It was low: just what Aldo Ianni, the then-director of the Canfranc Underground Laboratory, was hoping for. Now that sunken lead was being offered to any physics laboratory willing to pay 20 euros per kilogram—a fairly high price—for it.

Lead is mined and refined all over the world, but that centuries-old lead, sitting in a shipwreck, has a rare quality. Having sat deep underwater since before the United States of America was born, its natural radioactivity has decayed to a point where it’s no longer spitting out