U.S. prosecutors recently seized a 2,300-year-old vase, painted by the great Greek artist Python, which had long been on display at the Metropolitan Museum of Art. Evidence showed that tomb raiders had looted it in Italy in the 1970s. It is expected that the vase will ultimately be returned to Italy, as was the Euphronios Krater, another Greek vase that was illegally excavated in Italy in 1971, bought by the Met in 1972, and returned to Italy in 2008. Both vases passed through the hands of Giacomo Medici, an Italian art dealer who, in 2004, was convicted of dealing in looted antiquities, many of which ended up in the collections of museums.

How is it possible for looted objects to find homes in esteemed cultural institutions such as art museums, and what have governments done to try to stop this? Here is what you need to know about the dark side of the art world and the attempts to fight what goes on there.

This is how looted antiquities are laundered 

The plundering of archaeological sites is a common problem afflicting archaeology-rich countries, especially in the developing world. The clandestine excavation and removal of antiquities can cause enormous and irreparable damage to the looted sites, destroy much of the historical information that the antiquities carry, and rob communities of their cultural heritage. The looted antiquities then flow to rich countries, where they obtain a veneer of legitimacy and become museum exhibits or collector items.

Such laundering is possible, since most antiquities surface on the market and are sold without provenance: They carry no ownership history or information as to where and when they were found. The secrecy that typically shrouds the identities of buyers and sellers in the art market further complicates any attempt to trace an antiquity to its source. Excavated and exported illegally, antiquities often change hands several times before dealers and auction houses sell them to museums or private collectors. Any details of the objects’ illegal origin are erased or lost in the process: They often cannot be recognized as looted.

Unsurprisingly, art-trade practitioners try to minimize this problem, arguing that many unprovenanced antiquities come from legitimate sources and are not necessarily looted. Yet archaeologists identify the art market as the culprit and denounce the no-questions-asked approach that prevails there: the failure to verify the legitimacy of antiquities and the willful blindness to their dubious origins. According to archaeologists, the indiscriminate acquisition of unprovenanced objects, especially by U.S. art museums, fuels demand for plundered antiquities and hence encourages looting.

There is resistance to international cooperation

Many of the “source countries” where objects originate have laws saying that all antiquities found within the national territory are under national ownership and cannot be exported. Because source countries have failed to enforce the export prohibition, they want the market countries where antiquities are sold to prevent the import of looted material, shifting the burden of curbing the illicit antiquities trade.

In the 1960s, under the auspices of UNESCO (the U.N. agency that coordinates international cooperation in cultural matters), source countries initiated an international agreement that imposes significant controls on the movement of antiquities. This initiative, however, got a skeptical response from market countries, including the United States, Britain and Switzerland. These countries didn’t want to bear the costs of controlling imports of antiquities and wanted to protect their art markets, into which the looted antiquities flowed.

Therefore, market countries initially refused to join the agreement signed in 1970: the Convention on the Means of Prohibiting and Preventing the Illicit Import, Export and Transfer of Ownership of Cultural Property. In 1995, another international agreement was signed, which was intended to facilitate legal action for the return of looted antiquities. Market countries shunned that agreement as well, viewing it as a threat to their art markets.

Even still, cooperation evolved over time

The United States was the first market country to reverse its early opposition to international antiquities regulation  and join the 1970 UNESCO Convention. My research shows that this transformation was the product of public scandals, which exposed the unethical practices of American museums and collectors, as well as the efforts of the U.S. archaeological community. Archaeologists educated policymakers about the destruction that the illicit antiquities trade causes — and how the American art market fuels it. They urged U.S. participation in the UNESCO Convention to save the world’s cultural heritage and improve the United States’ image.

Yet policymakers also came under pressure from antiquities dealers and art museums. Those vehemently opposed the convention, arguing that it undermines the public interest in the enjoyment of art and unjustifiably asks the United States to solve other countries’ problems. Given this opposition, although the United States did join the UNESCO Convention in 1983, it implemented it in a limited fashion: Import restrictions on antiquities are imposed selectively, through bilateral agreements with source countries or on an emergency basis.

It took two decades for other market countries to follow the United States’ example. In 2002, Britain joined the UNESCO Convention after a series of scandals that called attention to the questionable practices of the London art market and its involvement with looted material. Other market countries, including Japan, Switzerland  and Germany, joined the convention in following years. Ultimately, these countries came to accept the principle that they had rejected early on: their responsibility to contribute to the prevention of looting through controls on the movement of antiquities.

Pressure on museums is growing

In recent years, the acquisition of antiquities by U.S. art museums has diminished. U.S. import restrictions — which many in the art community regard as too strong — have reduced the supply of antiquities. Museums caught unlawfully in possession of antiquities might become embroiled in legal battles over ownership and possibly even face criminal charges. The Association of Art Museum Directors has published guidelines that require museums to ensure the legitimacy of the antiquities they acquire. Media attention to the problem of looted antiquities has contributed to growing public awareness and concern, which pushed museums to behave more responsibly and made them susceptible to the pressure of source countries — such as Italy’s demand for the return of the Euphronios Krater.

Archaeologists maintain that, notwithstanding changing policies and practices, the U.S. art market continues to be a catalyst for looting. Yet it is clear that the ethical environment has changed fundamentally: No longer operating on the basis of the free movement of antiquities, the art world has come to accept significant constraints that are necessary for the protection of the world’s archaeological heritage.

Asif Efrat is an associate professor of government at the Interdisciplinary Center (IDC) Herzliya, Israel.