An underwater drone dove into Pavlopetri, the world’s oldest sunken city, off Greece’s coast. It found something terrifying.

The ruins of Pavlopetri lie in about three meters of water off the shore of Ponda Beach at the western end of Vatika Bay, in the southeastern corner of Laconia in southern Greece. The site sits in the narrow sea channel between the mainland and the island of Elafonisos, a short drive from the town of Neapoli. The remains cover roughly five hectares of seabed, about the size of seven football field…