EXETER, ENGLAND—Bill Hanson of the University of Glasgow and Ioana Oltean of the University of Exeter examined declassified spy satellite images of Romania and discovered traces of a 37-mile-long series of second-century A.D. Roman walls and fortifications that stretched from the Danube River to the Black Sea. “This new stretch of wall indicates that the area has been far more militarized and carefully controlled during the Roman period than was previously appreciated,” said Oltean. Some sections of the wall were 28 feet wide and more than 11 feet tall, but they were eventually forgotten and damaged by plowing and construction.