Researchers from the University of Western Australia have carried out a wide-ranging investigation over the past year, involving aerial surveys conducted by helicopter, ground survey and excavation and examination of satellite imagery.

In findings published in the Holocene journal in December, they said the “funerary avenues” spanning large distances in the northwestern Arabian counties of Al-‘Ula and Khaybar had received little examination until quite recently.

“The people who live in these areas have known about them for thousands of years,” researcher Matthew Dalton told CNN. “But I think it wasn’t really known until until we got satellite imagery that just how widespread they are.”

Dalton said the funerary avenues, which he had seen from a helicopter, stretched for hundreds, “maybe even thousands of kilometers” and that the same routes were often followed by those traveling along the main roads of today.

“Often you’ll find main roads tend to follow the same routes as the avenues because they tend to be the shortest route between between the two places they’re going to,” Dalton said. “And actually, in some cases, the the tombs themselves are so dense that you can’t help but walk on the ancient route itself, because you’re sort of hemmed in by the tombs.”

The tombs themselves are mostly either pendant-shaped or ring burials. Ring tombs involve a cairn surrounded by a wall of up to two meters in height, while pendant tombs have “beautiful tails.”

Using radiocarbon dating, the researchers determined that a concentrated group of samples dated back to between 2600 and 2000 BC, although the tombs continued to be reused until around 1,000 years ago.

“These tombs are 4,500 years old, and they’re still standing to their original height, which is really unheard of,” researcher Melissa Kennedy told CNN. “So I think that’s what particularly marks Saudi Arabia out from the rest of the region — just the level of preservation is unbelievable.”

Kennedy believes either single individuals or small groups were buried in the tombs, and the team have observed around 18,000 tombs along the funerary avenues while 80 of those have been sampled or excavated for research.

The researchers think the use of the routes long preceded the tombs, and are still not sure exactly why the tombs were built along the route — although Kennedy pointed to similar customs linked to land ownership in Greece and Rome in later history.

“A way of showing ownership perhaps, could be one reason the tombs were built,” Dalton said. “And there may be an element of, you bury your nearest and dearest alongside the route, because you’ll be passing them frequently, and you have a place to remember them.”

The next step for the team will be to do more radiocarbon dating and go back out on the field, before analyzing their data. And more discoveries are likely to follow, with Dalton saying the avenues may even stretch across into Yemen, especially as similar tombs are found in both that country and northern Syria.

“The third millennium is such an important period of time,” Kennedy said. “It’s when the Pyramids are built. And it’s where lots of different cultures are interacting with each other for the first time on a wide scale. So to see the appearance of this monumental funerary landscape in this period is really exciting. And huge new avenues of research to basically follow.”