The massive stone wharf loomed over Rio de Janeiro’s harbor, serving as the arrival point for nearly a million enslaved Africans during the first half of the 19th century. Today, the ruins of the Valongo Wharf are the only physical remains of a slave trade port left standing in the Americas.

On July 9, the United Nations Educational, Scientific and Cultural Organization (UNESCO) named Valongo Wharf a World Heritage site, along with northwest England’s Lake District, the walled city of Ahmedabad in India and the sacred Japanese island of Okinoshima, which doesn’t allow women on its shores.

Of the new sites on the list, none has a darker past than Valongo Wharf. Built in 1811, the wharf functioned as South America’s leading slave port. It was here that as many as 900,000 enslaved African men, women and children were held before being sold on the Brazilian slave market. Those who didn’t survive the ordeal—who died during the transatlantic journey or sometime after arriving at the wharf—were buried in mass graves nearby.

Valongo Wharf operated until the 1840s, when it was buried under newer docks built to welcome Teresa Cristina Maria de Bourbon, the new bride of Brazil’s Portuguese emperor, Dom Pedro II. Now both wharfs, covered by landfill after Brazil outlawed slavery in the late 19th century, lie near the Pedra do Sal, the historic center of Rio’s “Little Africa” neighborhood, which many former slaves came to call home.

As the last nation in the Western Hemisphere to ban the slave trade, Brazil has a long and painful history of trafficking in humans. Brazil didn’t put an end to the slave trade until 1888, a full 25 years after the Emancipation Proclamation. Over the course of 300 years, from the 16th to the 19th centuries, the country imported around four million slaves (some 40 percent of the total transatlantic slave trade). The overwhelming majority came from the former Portuguese colony of Angola, located on the Atlantic coast of southern Africa. Today, Brazil is home to more people of African descent than any country outside of Africa.

People celebrate after Valongo Wharf was added to the list of UNESCO World Heritage sites, in Rio de Janeiro, Brazil. The committee of the UN cultural body, meeting in Krakow, Poland, said Valongo was a reminder of an estimated 900,000 African slaves who were brought there by traders starting in 1811.(Credit: Maruo Pimentel/AFP/Getty Images)

Archaeologists didn’t uncover the remnants of Valongo Wharf until 2011, during the large-scale construction and renovation efforts leading up to the 2014 World Cup and the 2016 Summer Olympic Games in Rio. In addition to the other structures and artifacts unearthed near the Praça XV de Novembro—a square that housed Rio’s original slave auction site—they found a mass burial pit containing thousands of skulls and other bones.

At the time, politicians and activists in the the Afro-Brazilian civil rights movement known as Movimento Negro urged city leaders not to ignore Rio’s complicated past in their rush to construct a glittering new future. As Sonia Rabello, a legal scholar and former city councilwoman, told The New York Times in 2012: “We’re finding archaeological sites of global importance, and probably far more extensive than what’s been excavated so far, but instead of prioritizing these discoveries our leaders are proceeding with their grotesque remaking of Rio.”

Movimento Negro and others have long called for a memorial at Valongo Wharf to commemorate its dark history. After the wharf was uncovered, a young American landscape architect named Sara Zewde designed a monument that she and her supporters tried unsuccessfully to incorporate into the multi-billion-dollar revitalization of the area around the city’s harbor.

The future of the Valongo Wharf memorial may be uncertain, but thanks to the site’s new designation as a World Heritage site, UNESCO will begin working to preserve “the most important physical trace of the arrival of African slaves on the American continent,” according to the organization’s official announcement.

As historian Katia Bogea, head of Brazil’s national heritage institute (IPHAN) argued before the UNESCO committee, honoring the slave wharf serves “to make us remember those parts of the history of humanity that must not be forgotten.”