Research by archaeologists at the Universities of Glasgow and Exeter has identified a long wall that ran nearly 60 miles from the Danube to the Black Sea over what is modern Romania. It is considered the most easterly example of a man-made frontier barrier system in the Roman Empire.

Built in the mid-second century AD, 'Trajan's Rampart' as it is known locally, once stood 8.5m wide and over 3.5m high and included at least 32 forts and 31 smaller fortlets along its course. It is thought to have served a similar purpose to other Roman frontier walls, such as Hadrian's Wall, built to defend the Empire from threats to the borders.

Trajan's Rampart actually consists of three separate walls of different dates; the 'Small Earthen Wall', the 'Large Earthen Wall' and the 'Stone Wall'. The constructions were previously known about, although wrongly thought to date to the Byzantine or Early medieval period.

Although it is estimated that over 50% of all archaeological sites in the UK have been discovered from the air, other countries are less well studied. Archaeologists believe that studying declassified photographs taken during covert surveillance may herald a new era for archaeological discovery, and may help to uncover and identify thousands of new archaeological sites around the world.

Tens of millions of images of Europe and the Middle East were taken by Allied and German airforces during the First and Second World Wars and are now held in vast public archives. Alongside this, a considerable historical aerial resource is also now available from the recently declassified covert US CORONA satellite intelligence programme of the 1960s, 70s and 80s, which includes around 900,000 photographs from around the world.

These images are particularly valuable to modern archaeologists as they effectively turn back the clock to a time before later Twentieth Century development changed the face of the landscape through industrialisation, intensive farming practices and urban development.

Bill Hanson, Professor of Roman Archaeology from the University of Glasgow, said: "We believe we have enough evidence here to demonstrate the existence of a chronologically complex Roman frontier system, and the most easterly example of a man-made barrier in the Roman Empire, serving to block an important and strategically valuable routeway. It is an incredibly important discovery for the study of Roman history."

Dr Ioana Oltean, Senior Lecturer in the Archaeology Department, University of Exeter, said: "Photographs from military surveillance are revealing more than those who took them could have imagined because now, half a century or more later, they are proving to be of enormous benefit in showing us our lost archaeological heritage. Thanks to such images, the landscape of this frontier zone is now known to have been as busy in the past as it is today. We hope that this discovery will provide stimulus for further examination of many more neglected frontiers."

 Archaeologists from two United Kingdom universities examining declassified spy photos rediscovered part of a what they believe is a series of Roman fortifications dating back to the 2nd century A.D.

Although parts of the ruins had once been known to 19th-century researchers, they were subsequently misidentified, dismissed and largely forgotten, according to Bill Hanson, a professor of Roman archaeology at the University of Glasgow. In some areas the structures were heavily damaged by ploughing or construction -- even to the point of complete destruction.

"If you look at any modern book on Roman frontiers, you will find no mention of [these fortifications]," Hanson told The Huffington Post. "[They have] kind of disappeared from consciousness."

While the ruins may have lost their significance, many hundreds of years ago they were an impressive monument to the power of the Roman Empire. The entire structure ran about 37 miles across modern-day Romania from the Danube River to the Black Sea. In some parts, the system of walls and forts may have once stood 28 feet wide and more than 11 feet high, according to the researchers' best guesses.

Heavily patrolled by Roman soldiers, Hanson told HuffPost that this "easternmost linear barrier of the Roman territory" would have been "without a doubt the most densely garrisoned linear barrier anywhere in the Roman empire." While dating the ruins is inexact without the results of formal excavation or carbon dating, Hanson added that he believes construction most likely began in the 2nd century A.D.

Dr. Ioana Oltean, a senior archaeology lecturer at the University of Exeter, told The Huffington Post in an email that she began the project as a way to systematically map any and all archaeology visible from the air and from satellites and "to use this data to understand the impact of the Roman conquest on indigenous landscape and communities."

"I was particularly keen to consult early photographs pre-dating modern development," Oltean told HuffPost. "I knew that the landscape in that region had undergone significant development during the communist regime and this development could have wiped out some archaeological sites without them being necessarily mapped or properly investigated."

Oltean and Hanson therefore set out to reassess the area, relying heavily on photographs to guide them where erosion or development had obscured traces of the ancient barrier.

A release from the University of Glasgow notes that the researchers were tapping in to a vast archive of declassified aerial pictures taken during World Wars I and II, as well as declassified images made by American spy satellites during the Cold War. In the case of the Roman fortifications, Oltean told HuffPost that digitized copies of aerial photographs made on May 31, 1944, were particularly useful in pointing out never-before catalogued forts along the route.