ST DAVIDS, WALES—A woman walking along Caerfai Beach during low tide spotted three submerged mines dating to the First World War. “At first I thought it was some sort of Roman pottery,” said Julia Horton Powdrill, who happens to be an archaeologist. “Personally I think the sea has been so rough and swirly recently that it’s likely the sand covering them has been washed away,” she added. Powdrill had been out foraging for seaweed when she made the discovery.