The structures were found in the area of Harrat Khaybar, a volcanic field in Saudi Arabia. Those odd-looking domes you see were made from eruptions that happened in the water, which was a very long time ago. According to scientists, it's been quite a while since the last one —around 600-700 AD.
Australian archeologists examined the area and found around 400 structures draped across different domes. They also described the walls of these structures as "stone built, roughly made, and low." They are easily the oldest man-made structures in the area, but when they were actually made is up for debate.
They could have been built around the time of the last eruption — pieces of lava were found on top of some of them. That makes them at least 1700 years old, but there's more evidence that it might be older. Researchers said that this could have been constructed during a time when the area was wetter, which means they could have been constructed 9,000 years ago.
The gates form a rectangular design most of the time, but sometimes they have this sort-of "I" shape. They aren't gonna be walking tours anytime soon - structures like these are almost always found in inhospitable areas with very litter water of vegetation. That's what helped them from keeping them in one piece throughout thousands of years.
Saudi Arabia has a pretty big history with archeology and it shows with the kind of discoveries people have stumbled into. WWII RAF pilots flying overhead discovered structures they thought looked like kites because of their triangular, arrow-like design. The kites might have been used as really big funnels to corral animals for hunters way back in the day.
Other shapes found from images came out as even weirder than animal-gathering kites. The archeologists that discovered these called these structures "comets" or "triangles" from their quirky circle/triangle combination. The tails of those comets come in different lengths, just like the gates.
Over in nearby Jordan, there are rock formations that were also found in, you guessed it, a lava field. Known to the Bedouin as the "works of the old men," they are round instead of rectangular. Both are buried in lava flows, which is a sign that they might be pretty old.
What really helped professionals in these new discoveries is using the same thing you use looking up your name for fun. Since the launch of Google Earth, there has been a huge boost in sky archeology from people just looking at images. You'd be surprised what you can find when bored out of your skull just from staring at the earth from above.
Oddly enough, the gates were first found by people that had absolutely nothing to do with archeology. Volcanologists were out there mapping out the lava domes of Harrat Khaybar in the 1980s when they stumbled onto them. They were more interested in researching at domes like Jabal Abyad (the one in the back of this image), which means "white mountain" in Arabic.
The ground view of Sammah Gate 31 shows just how difficult it is to see the gate up close. From hundreds of feet in the air, you see beautiful gates. But here are the low-level piles of rocks that stretch the volcanic sand.
An amateur archeology group took the Google Earth images and made their own expedition into Harrat Khaybar. During their 2008 visit, they were amazed at the strange geometric rubble, some that they believed are the remains of tombs. Their report on the gates is pretty cool (note: the site is in Arabic but the English translation can be understood rather well.)
The problem is, neither the pros or the amateurs have any idea why hundreds of structures are built in the middle of nowhere of Saudi Arabia. Besides the kites used to hunt animals, there isn't enough evidence to know for sure what the purpose is for the rest of them. Remember, everyone is basically working on images they're ripping from Google, not rock-solid evidence from the gates themselves.
The Australian research team will publish their findings in December of 2017. Until then, they're going to have to go on the ground floor and really dig into the history of the stones. With enough carbon dating, maybe they'll finally solve the mystery of the gates of Harrat Khaybar.