ENZO SUMA WAS STROLLING ON a crescent-shaped sandy beach in Puglia, the heel of Italy’s boot, when his eye fell on a strange-looking object in the sand a few feet away. It was winter 2018, and Suma, a 40-year-old naturalist and guide, had taken part in several beach clean-ups before; he was no stranger to the waste that accumulates on Mediterranean shores in the winter.

But this object, a cylindrical plastic sunscreen bottle, stood out. Its colors—white, green, blue, orange—seemed faded, and the instructions were abraded, suggesting that it had spent a long time at sea. Suma noticed that the bottle had been priced in lira, Italy’s currency before the introduction of the euro in 2002. “Liras?” he thought, dumbfounded.