BOSTON, June 29 (Reuters) - Iraq's archeological sites, despite protection efforts, are so ravaged by looters that the pillaging has landed the entire embattled nation on a list of the world's 100 most endangered cultural sites.

Two years after the U.S.-led invasion of Iraq and a widely publicized break-in at the Baghdad Museum, the country is a hotbed of antiquities plundering that threatens to put huge gaps in the understanding of its rich history, experts say.

Once called Mesopotamia, Iraq is regarded as the cradle of civilization and the birthplace of cities. This year's World Monuments Fund's list of the world's 100 most-endangered sites named Iraq, the first time an entire country was listed as at risk.

"It's devastating. It's obliterating the country's heritage, and we might never know the full story of Iraq," said Clemens Reichel, an archeologist at the University of Chicago. "Archeological sites now look like lunar landscapes."

The ancient Assyrian capital of Nineveh, the ziggurat at Ur, the temple precinct of Babylon and the 9th century spiral minaret at Samarra have been "scarred by violence," the World Monuments Fund said, while adding that looting has damaged other equally significant sites, especially in the South.

Touring Iraqi sites from a U.S. military helicopter, McGuire Gibson, a professor at the Oriental Institute at the University of Chicago, said he could see between 250 and 300 men digging at the site of the ancient city Adab. "The damage was unbelievable," he wrote in a paper.

The pillaging could hurt a wider understanding of human history, said Donny George, director of the Iraq Museum.

"What was stolen is not just Iraqi heritage," said George, recently in Boston to promote a book on the museum's collection entitled "The Looting of the Iraq Museum, Baghdad."

"It is the heritage of mankind, the origins of agriculture, animal husbandry, so when you lose such material, even just one piece, it's a great loss," he said.

BLACK MARKET

In spite of efforts by UNESCO, the cultural arm of the United Nations, and law enforcement including the FBI and Interpol to curb ransacking and the trafficking of artifacts, Iraqi pieces chronicling millennial of human history are finding their way to private collections across the world.

Archeologists worry that once removed from their surroundings, relics become almost impossible to understand in context. Once unearthed, they are smuggled out of the country through Jordan, Lebanon and Turkey to Europe and the United States and flood the black market.

Some pieces such as cylinder seals -- small engraved stone cylinders used to stamp impressions on wet clay that once fetched small fortunes at auction -- sell on the online auction site eBay for a few hundred dollars.

In the past two years, about 1,000 objects were confiscated at airports in the United States, and hundreds more were found in Europe and neighbors of Iraq such as Jordan and Syria.

In spite of the international effort, Reichel, who coordinates a clearinghouse of missing Iraqi antiquities designed to help in their recovery, said more could be done.

"What they are doing now is like waving your hands around to try and catch flies," he said. "One thing they could do is forbid the trade of Iraqi antiquities."

UNESCO sends information on missing artifacts to Interpol, which tries to track them down. The U.N. group also has trained and equipped border patrol and site guards to ensure relics are not smuggled out of the country.

TRADITION OF LOOTING

Conflict and looting have historically gone hand in hand, although motivations may have changed through the years.

"In ancient wars, getting the statue of a king would have been an act to insult the conquered country or city," George said. "Now it's about the material value."

Tomb raiders and looters abound from Latin America to Asia and the Middle East. In Israel for instance, a land rich in ancient history, thieves raid about 300 antiquity sites every year and hundreds more in the nearby West Bank.

During the 1991 Gulf War, looters broke into nine of Iraq's regional museums, stealing more than 4,000 objects from statues to clay tablets and pottery. Less than a handful of those artifacts has been recovered.

The thieves who stole 15,000 pieces from the Baghdad museum as U.S. troops took the city in April 2003 were believed to be professionals with inside knowledge of the museum and archeology, George said.

Using glass cutters, they entered the building and carefully picked ancient relics from replicas and found a storeroom that held boxes full of precious jewels.

About half of the pieces have been returned to the museum, including the Warka Vase, an alabaster piece from 3000 BC, and the marble Warka Head, also more than five millennial old.