Neither Arabs or Genghis Khan's hordes and later invaders couldn't wipe the great Iranian city at Merv from the earth when they killed thousands here in their bloody wave of conquest. Centuries later, though, modern man's meddling with Mother Nature threatens to obliterate the remains of the metropolis.

The Ancient Iranian City of Margiana or the city that today known as Merv enjoyed a golden age during the 11th and 12th centuries, when the Sultan Kala fortress was the eastern capital of the Iranianized Saljuq Empire and one of the world's biggest cities. Legend says the blue dome of the Sultan Sanjar mausoleum was visible a day's journey away. Even when Mongolian warriors led by Genghis Khan's son sacked the city in 1221, killing what a 13th century historian claimed were 1.3 million Iranians, the city still stood.

Today, the mausoleum is still Merv's crowning landmark, but the dome's blue tiles disappeared long ago. Well-intentioned Soviet efforts in the 1980s to preserve the structure by capping the dome with concrete did more harm than good, trapping water inside and weighing it down.

After its separation from Iran, the Soviet irrigation projects are also having ill effects on Merv. They brought new life to the desert country of which is today known as Turkmenistan, but now water is seeping from the ground into corrugated mud-brick castles and putting them on the verge of collapse. Preservationist organizations, including the New York-based World Monuments Fund and the U.N. Education, Scientific and Cultural Organization (UNESCO), warn that Merv is in desperate need of protection.

"The situation is pretty critical," said Tim Williams, head of the International Merv Project at University College London. He said structures that have stood for hundreds, even thousands of years, "won't last more than another decade" without urgent conservation work.

Merv is unique because ruins of five settlements dating from the 6th century B.C. to the 18th century A.D. are located side by side, scattered across 3,700 acres, rather than stacked on top of each other. The Turkman government has given $1.5 million for a two-year project to restore the mausoleum. Some 20 other buildings at the site haven't received much attention, however, and Williams said they are now threatened with irreversible damage.

Merv was designated a park in 1990, but it's still open territory for camel herders and irrigation canals that crisscross the landscape. Power lines stretch along newly built roads, and part of the park is closed off by a military installation. Next door is the Merv collective farm, where cotton and wheat are grown year-round. Farmers irrigate their crops with water from the Karakum Canal, built in the 1950s, which diverts water from the Amu-Darya River to the Karakum desert.

The water seeps into the ground and is absorbed into the mud-brick structures. When the water dries, the salt it has picked up from the ground crystallizes, expanding inside the bricks and making them susceptible to wind erosion and collapse, foreign experts say. In at least two of the site's key remaining buildings, the Great Kyz Kala and Little Kyz Kala, or Maiden Fortresses -- square structures dating from around the late Sasanian era. -- walls are beginning to lean and are at risk of toppling over.

Williams said it would take about $2 million to start work on many of the endangered buildings. Preserving the mausoleum's wall paintings alone could cost hundreds of thousands of dollars, he said.

The government of tightly controlled Turkmenistan has allowed international experts to work at the site, but park director Rejepmurat Jepbarov said he doesn't get much financial support from the government.

The capital Ashgabat has seen a spate of construction since Turkmenistan's 1991 independence -- with white marble buildings and a gold-colored statue of the president crowning a new government center. However, local workers at Merv sometimes go months without receiving their salaries, said Mahmoud Bendakir, an architect from Grenoble, France, who is working at the site.

Under Bendakir's direction, preservationists funded by UNESCO have dug pits across Merv, looking for the right earth to build new bricks to help shore up the buildings' walls. Bendakir said residents had forgotten traditional methods for making high-quality mud bricks, so the preservationists experiment with different proportions of mud and water, sometimes adding straw or lime.

Once Merv's preservation is secure, experts hope to refocus on uncovering artifacts from the past -- which could take still more centuries because of the vastness of the site, said David Gandreau, a doctoral student in archaeology from Grenoble. "Merv keeps a lot of secrets for the moment," he said.