Almost 400 mysterious stone structures dating back thousands of years have been discovered in Saudi Arabia, with a few of these wall-like formations draping across old lava domes, archaeologists report. Many of the stone walls, which archaeologists call 'gates' because they resemble field gates from above, were found in clusters in a region in west-central Saudi Arabia called Harrat Khaybar.
     Discovered mainly through satellite images, a few of the gates are actually located on the side of a volcanic dome that once spewed basaltic lava, researchers found. The gates "are stone-built, the walls roughly made and low," David Kennedy, a professor at the University of Western Australia, said. The gates "appear to be the oldest man-made structures in the landscape," Kennedy noted, adding that "no obvious explanation of their purpose can be discerned."
     The smallest of the gates extends about 43 feet (13 meters), while the longest is 1,699 feet (518 m) long. Many have multiple stone walls that, in some instances, form a rectangular design; some of the others, called 'I' type gates, have only one stone wall with heaps of stone at each end.
     "Gates are found almost exclusively in bleak, inhospitable lava fields with scant water or vegetation, places seemingly amongst the most unwelcoming to our species," Kennedy wrote. Other types of stone structures - such as "kites," which were used to hunt animals, and "wheels," named for their shape - have also been discovered in these lava fields.
     The kites, wheels and other types of stone structures were typically found to be built on top of these gate-like walls, suggesting that the gates predate these stone structures, Kennedy said. The remains of lava flows are also sometimes found on top of the gates, indicating that the gates are also older than some of the flows, Kennedy said.
     In the 1980s, before the discovery, volcanologists Vic Camp and John Roobol mapped an area of the Harrat Khaybar that included a lava dome festooned with gates and other stone structures. Camp estimates that some of the gates around the lava dome were built around 7,000 years ago. Archaeological fieldwork is necessary to determine what the gates are and when, exactly, they date to, Kennedy said.

Edited from LiveScience (17 October 2017)