Who built these mysterious ‘hellgates’?
ARCHAEOLOGISTS know these stone structures are thousands of years old.
That's about it.
They're deep within Saudi Arabia's deserts.
They were detailed through analysis of satellite photographs by University of Western Australia researcher David Kennedy.
And they're only called 'gates' because - from the air - they simply look like line drawings of a traditional gate found of farmers' fields.
That's because these 'gates' make no sense.
All of them appear to lead to ancient domes of lava.
And at the time the rock walls were put in place, the lava was most likely active.
GATES TO HELL
The discovery is in the volcanic region of Harrat Khaybar, in Saudi Arabia.
And archaeologist have a lot of work ahead of them to establish their exact age, what their purpose was - and who may have built them.
But at the time of their construction, Harrat Khaybar would have been a dramatic place.
The domes would have been bubbling with basaltic lava and noxious fumes.
Periodically, outpourings of lava would have flowed over the sides of the domes to create fields of steadily cooling molten stone.
It's upon these that the low, rough stone walls are piled.
"Identification, mapping and preliminary interpretation imply an early date in the sequence of the works-perhaps the very earliest-but no obvious explanation of their purpose can be discerned," Professor Kennedy writes.
"Gates are found almost exclusively in bleak, inhospitable lava fields with scant water or vegetation, places seemingly amongst the most unwelcoming to our species."
But a clue may lay in their location.
It's not the first time structures have been found in subh places.
"The lava fields are often rich in archaeological remains, implying a moister past and more abundant vegetation, and recent fieldwork identifying larger settlement sites supports this notion," Professor Kennedy writes.
"As in the much better explored lava field of Jordan there are many thousands of stone-built structures which are collectively known to Bedouin as the 'works of the old men'."
Some of the stone walls are actually buried by the lava flows - a sure indicator of their age.
They're not high enough to restrain livestock.
They're also not exactly uniform. Some are roughly in the shape of an "I". Others are rectangles.
Most are about 200m long. The largest extends for 519m.
The researchers, who will publish their findings in the journal Arabian Archaeology and Epigraphy, have found photographic evidence of other structures in the area.
Which indicates there may be much more lurking under the shifting sands, rubble and lava flows.
Among their satellite-photo finds are stone 'kites' - corrals of rocks likely used to guide animals into traps. There are also enclosed 'wheel-like' structures, and what look to be stone burial mounds (cairns).
"The Works known as Kites-which are certainly animal traps, may be as old as 9000 years before present in some cases and there is one example of a Kite overlying a Gate (ie the Gate is older). So Gates may be up to or more than 9000 years old, which takes one back to the Neolithic," Professor Kennedy says.
Only feet and eyes on the ground will take this discovery further.
But the biggest challenge archaeologists face will simply be getting to them, given the site's remoteness and hostility.
Samples will need to be collected from the lava fields and tested with techniques such as carbon-dating and optically-stimulated luminescence to determine their age.
"The availability of high-resolution satellite imagery of Saudi Arabia on publicly available platforms such as Google Earth and Bing Maps has been transformational for archeology," Professor Kennedy writes.
"Within just a few years tens of thousands of sites previously unrecorded and scarcely known to the academic world have been mapped."
He is a specialist in using satellite and aerial photography to study ancient landscapes. His Aerial Photographic Archive for Archaeology in the Middle East is a project intended to investigate archaeological sites around the Arabian area using remote sensing.
The 'gates', he says, were first found by members of the public browsing satellite photos.
"This novel site type was first brought to a wider audience by a group of Saudi nationals - all non-archaeologists - who have been engaged in exploring the cultural heritage of their country," Professor Kennedy writes.
"In particular, Dr Al-Sa'eed, a medical doctor who, together with other members of what they have called the Desert Team, used Google Earth to examine parts of the landscape, visit some of the sites, and illustrate them on a website."