Today it's a mirage-like expanse of monumental ruins. But under the Roman Empire, Palmyra was a trading metropolis, according to historical and archaeological evidence.

Despite nearly a century of research, though, a key question remains unanswered: How did this city of 200,000 thrive in the middle of an infertile Syrian desert?

Once a required stop on caravan routes that brought Asian goods west to eager Romans, Palmyra (map) has "always been conceived as an oasis in the middle of the desert, but it's never been quite clear what it was living from," said Michal Gawlikowski, the retired head of the University of Warsaw's Polish Mission at Palmyra.

And what an oasis: Among the ruins are grand avenues lined with columns, triumphal arches, and the remains of an ancient market where traders once haggled over silk, silver, spices, and dyes from India and China. (Download Palmyra wallpaper.)

To find out what made it all possible, archaeologist Jørgen Christian Meyer began a four-year survey of the 40 square miles (104 square kilometers) just north of Palmyra in 2008. The area was targeted for its mountainous terrain, which channels precious rainwater to otherwise dry streambeds—making the region marginally less hostile to agriculture.

Through ground inspections and satellite images, the archaeologists eventually found outlines of more than 20 farming villages within a few days' walk of the city—adding to about 15 smaller settlements previously uncovered by other researchers to the west of Palmyra.

Crucially, the researchers also found traces of extensive networks of man-made reservoirs and channels to capture and store the rainfall from sudden, seasonal storms, said Meyer, of the University of Bergen in Norway.

The landscape around the city, it now appears, was intensively farmed and most likely included olive, fig, and pistachio groves—crops known in the region from Roman accounts and still common in Syria. Barley too was grown, according to a pollen analysis Meyer's team conducted on a mud brick from the survey area.

Nature or Nurture?

It may be tempting to pin Palmyra's shifting agricultural fortunes on climate change, Meyer said, but his money is on human ingenuity.

"There has been a very intense discussion about climate changes since antiquity, and some researchers use this as an explanation of almost everything—the fall of empires, etc.—but there is more or less consensus that the macroclimate has not changed dramatically since antiquity."

Though the area north of Palmyra is a dry steppe, Meyer added, it has "huge potential for farming if you invest the time and energy in controlling the resources."

The ancient residents, he estimated, managed to capture and channel the 5 to 6 inches (12 to 15 centimeters) of annual rainfall. The system lasted until about the year 700, archaeological evidence suggests—roughly the post-Roman period during which Palmyra began falling into ruin.

Tax Haven

The findings already appear to be changing how scholars look at Palmyra.

"This proves there were farms around Palmyra, and they were cultivating wheat and other grain," said the University of Warsaw's Gawlikowski, who wasn't involved in the new research.

"It's now clear they were feeding the city."

The new picture also helps explain the city's ancient prominence as a distribution hub, despite its difficult location.

After travelling from Asia via the Indian Ocean or Persian Gulf, Europe-bound goods were floated partly up the Euphrates River, then off-loaded and carried by caravan across Syria—via Palmyra—to Mediterranean ports. (See regional map.)

Other routes—floating goods farther north on the Euphrates to what's now Turkey, for example, or along the Red Sea and up the Nile—would have been more direct and might even have been faster.

So why cut across the desert on camelback?

The answer, project leader Meyer said, has everything to do with profits—and those newfound farms.

Two thousand years ago Palmyra was in a chaotic region sandwiched between the mighty Romans to the west and the Parthian and Persian empires to the east. Small kingdoms, each demanding payment, lined the Euphrates.

By contrast, the desert caravan hub likely offered a one-stop alternative to all the river taxes—and it was made possible largely by those surrounding croplands, Meyer said.

Local farmers, he theorized, cooperated with the nomadic herders who brought caravan-ready camels and sheep to Palmyra, allowing them to graze after the harvest.

That grazing, Meyer added, would have had a side benefit, as the animals helped fertilize the fields. "The nomads come with their herds and leave a small present, and get water in return."

Palmyra Archaeology: Casualty of War

The dusty outlines of reservoirs may fail to inspire the imagination—or the tourism trade—the way Palmyra proper's temples and amphitheater have. But archaeologist Cynthia Finlayson says Meyer's work is monumental in its own way.

When archaeologists focus only on "elite buildings, we miss what's going on in other parts of society," said Finlayson, a Palmyra scholar at Brigham Young University in Utah, who wasn't part of the project.

"Christian's work is really important. It's filling in a lot of gaps."

Even so, many gaps remain, and it's not clear when, or if, they'll be filled.

Meyer, for example, didn't have time to do any digging before armed conflict in Syria prompted his departure last spring. "We would like to have excavations to get a more detailed idea of the chronology," he said. "But as things look now, I don't know if that will be possible."

The violence and chaos in Syria threatens to do more than just delay research.

Finlayson, who was part of the last U.S. team to leave Syria in 2011, said the countryside around Palmyra has become home to well-armed antigovernment militias, leaving the government eager for armed assistance.