In the 1920s, Royal Air Force pilots flying airmail routes from Cairo to Baghdad noticed something bizarre in the lava fields of Syria, eastern Jordan and Saudi Arabia.
Dotting the bleak, barren desert landscape, hundreds of kilometres from anywhere, were thousands upon thousands of elaborate stone wheels, measuring up to 70 metres wide and visible only from the sky.
Flt Lt Percy Maitland documented the presence of the mysterious structures in a 1927 article for the archaeological journal Antiquity.
They remained largely a secret until the 1970s when Dr David Kennedy, now a professor of classics and ancient history at the University of Western Australia, saw them in great numbers while studying old survey photographs from Jordan.
Beginning in the mid-1990s, Dr Kennedy led an aerial photography project aimed at documenting Jordanian archaeological sites.
"These structures are largely unknown," he said. "Frequently, you can't see any of these structures from the ground. Or you can just see a jumble of boulders that don't make any sense. But you go up a small distance and they are extraordinary."
In acoming article for the Journal of Archaeological Science, Dr Kennedy reveals one of the most comprehensive studies on these stone structures, which stretch across the Middle East from Syria to Yemen and could number more than 1 million.
The structures, classified into sub-categories including wheels, rings, pendants and cairns, differ slightly across the region but most share striking similarities.
Wheels are "large circular or sub-circular enclosures, usually with thick walls, often with one or more internal partitions of equal thickness that resemble the spokes of a wheel"; while rings are small versions that can be internally divided or not.
Most numerous are the pendants, consisting of a head and a tail. In Jeddah, one uninterrupted tail spans 5km. Most of the structures are no more than 50cm high.
"They really dominate the landscape, suggesting a lot of effort was put into constructing these over a huge area," Dr Kennedy said.
Known by the Bedouin as "the works of the old men", they are at least 2,000 years old but could have been built up to 9,000 years ago, says Dr Kennedy.
Compared to the Peruvian desert's Nazca drawings - which date as far back as the year 400, number in the hundreds and have a maximum breadth of about 270 metres - the Middle East patterns are more numerous, bigger and much older.
"These volcanic lava fields are the last place you'd expect to find these kinds of structures," Dr Kennedy said. "The landscape is not hospitable. It looks bleak and barren. They're so unusual."
At least 3,000 structures have been found in Jordan and Dr Kennedy's recent research has documented nearly 2,000 in Saudi Arabia.
A specialist in Roman archaeology, he uses aerial photography gathered during annual trips to Jordan since 1997.
But for the Saudi study he used high-resolution images from Google Earth.
"What David is doing is pioneering work," said Dr Tobias Richter, an assistant professor in the department for cross-cultural and regional studies at the University of Copenhagen and an archaeologist also working in Jordan.
"With Google Earth, we've reached a position where you can do research from your desk in the UK or Australia, or wherever you can access it. It's a huge advance for us, but it's a tool that has to work together with going out into the field and looking on the ground."
Dr Kennedy scanned 1,241 square km of land near Jeddah - a strip 17km wide by 73km long - using Google Earth. He recorded six categories of site on the basalt lava fields.
But studying the thousands of sites raised more questions than it answered. In the Jeddah sample no kites, which number in the hundreds in Jordan, were found.
And still no one knows what they were for. The kites' function is largely guesswork. They might have been large traps for corralling oryx or gazelle.
It could be decades before the sites are all excavated and studied properly.
Dr Gary Rollefson, a professor in the department of anthropology at Whitman College in Washington state who also works on prehistoric sites in Jordan, has found hundreds of basalt structures.
Most are tombs and ritual buildings at two sites east of Azraq in Jordan, but there are also shapes similar to the ones studied by Dr Kennedy.
"There are three wheels at Wisad [Pools, one of the Jordan sites], which are really bizarre," Dr Rollefson said. "I don't know what those things are but the burials are made at the same time the wheels are.
"Really, we're just guessing. We don't know what they are but they are elaborate."
No one has yet undertaken a similarly detailed study of other GCC countries, although Dr Kennedy said he had not seen any structures in the UAE or Oman.
Comprehensive satellite or aerial images of the Gulf are not common, since most Middle East countries will not provide them for archaeological research and limit use of airspace. The ones that do exist are mostly of too low a resolution to make them useful for study.
Dr Kennedy plans to continue studying the Saudi satellite imagery for at least two or three more years to catalogue the structures and try to make sense of them.
For now just a handful of researchers are working in this remote area, but they expect others to join them as findings emerge.
"It is a very far removed area that is hard to get to," Dr Rollefson said. "We staked a spot in a region that no one wanted to go to. We're discovering a new world that nobody bothered to look at before."
And thanks to Google Earth, studying the desert of the Middle East will only become easier.
"With the advent of technology using satellite imagery and aerial photography, there has been a renewed interest in the role these marginal regions have played," Dr Richter said.
"There's a lot of key moments of human history that have happened in Arabia, and that's why it's such a fascinating place to work."