Archaeologists have spotted hundreds of mysterious, seemingly ancient man-made structures in the sands of Saudi Arabia on Google Earth.
The archaeologists came across the structures in in a region of the country called Harrat Khaybar.
Some are four times the length of a football pitch, and experts have suggested that they may be up to 7,000 years old.
Most popular on Yahoo News UK:
Couple caught on CCTV having sex in Domino’s takeaway spared jail
Congresswoman says Donald Trump told dead soldier’s widow: ‘He knew what he signed up for’
North Korea warns US military allies they face nuclear attack
NHS bans some obese and smokers from surgery ‘indefinitely’
The archaeologists refer to them as ‘gates’ because of the resemblance they bear to garden gates when viewed from above – but their purpose, and age, remains unknown.
David Kennedy of the University of Western Australia wrote, according to LiveScience, that the gates ‘appear to be the oldest man-made structures in the landscape.
He wrote: ‘Gates are found almost exclusively in bleak, inhospitable lava fields with scant water or vegetation, places seemingly amongst the most unwelcoming to our species.’
Some of the gates are sufficiently old that lava flows appear to go over them.
The gates are up to 1,700 feet long, Kennedy writes.
He has spotted hundreds of archaeological sites in the area, mainly using Google Earth imagery – as it’s difficult for researchers to access sites on the ground.