WARSAW, POLAND—According to a La Brújula Verde report, hundreds of grids carved into the ruins of the ancient Greek city of Ptolemais, in what is now northeastern Libya, have been documented by archaeologist Zofia Kowarska of the University of Warsaw. The grids are concentrated on the eastern side of the site and are thought to have been engraved after the Arab conquest of the region in the seventh century A.D., when Ptolemais was abandoned. “Sometimes in a single spot we find a dozen, even 20 or 30 boards right next to each other,” Kowarska said. The boards usually consist of a number of small, circular depressions arranged in a square or rectangle. Local people living now in nearby Tolmeita suggest...