The remodelling project at a 19th century home in Rio's old Gamboa district came to an abrupt halt.
Labourers digging in the yard to check the foundations had found human bones. Thousands of them.
The home-owner, Ana de la Merced Guimaraes, soon discovered that her house was sitting on the Cemeterio dos Pretos Novos - Portuguese for Cemetery of New Blacks.
Ten years later, the city wants to preserve the find as a rare window into Brazil's colonial past - and one of the darkest pages of its history.
Andre Zambelli, head of the Rio's Cultural Heritage Department, said: "It's certainly one of the city's most important discoveries."
He added that "it shows how the slave trade happened, confirms what's in textbooks, puts history in our hands."
Workers have recovered 5563 bone fragments and teeth, some rounded or carved in styles characteristic of people that lived along the Congo River in Mozambique and South Africa.
They also found pieces of fine English china, stoneware and African clay pipes, dishes and metal ornaments dumped in the graves as trash.
Rio consulted experts from New York, where the African Burial Ground was discovered in lower Manhattan during construction of a skyscraper in 1991, with the remains of at least 419 slaves or free blacks buried in colonial times.
Most poor Brazilians, who live in
slums, are Black, a report says
The US government designated the site a National Historic Landmark in 1993.
"It's the same connection, a re-encounter with African history, labour and culture," Zambelli said.
Rio believes its cemetery was bigger.
More than 20,000 bodies probably were buried there between 1769 and 1830, Zambelli said, but no one knows exactly because no records were kept. They were the bodies of slaves who died before they could be sold.
Brazil was the New World's biggest market for African slaves.
Of an estimated 10 million Africans brought to the Americas, nearly half came to Brazil, where they worked in gold and diamond mines, or on coffee and sugar plantations.
When Rio became Brazil's capital in 1763, residents soon began objecting to the squalid slave market in downtown streets, near the palace where the Portuguese royal family took up residence after fleeing Portugal ahead of Napoleon's invading army in 1807.
"It was Rio's holocaust. Few people know about it. We're re-discovering a story that was erased from history"
Municipal Council for the Defence of Black Rights
So, the market was relocated to the marshy Gamboa district, which became the unofficial graveyard for slaves after a Franciscan churchyard filled up. Bodies were piled in
stacks on the street and often burned before burial under a few shovelfuls of soil.
The treatment still rankles rights activists.
Marcelo Monteiro at the Municipal Council for the Defence of Black Rights says: "It was Rio's holocaust.
"Few people know about it. We're re-discovering a story that was erased from history."
Haidar Abu Talib, of the Muslim Charity Society, said many of the slaves buried in the cemetery were Muslims.
He said former slaves remained "invisible" even after slavery was abolished in 1888 and some Brazilians would like to keep it that way.
"When slavery ended, the government - run by the elites that always benefited from slave labour - wasn't concerned about making ex-slaves full citizens," Talib said at a ceremony for Black Consciousness Day.
"People ask me why I'm doing this, but the more I learn about how the Africans were abused and realise it's been forgotten, I swear they won't forget it here, not while I have the strength"
Ana de la Merced Guimaraes
"Even today, their descendants are victims of social injustice."
Although nearly half of Brazil's 183 million people are black or mixed-race, the country's cherished self-image as a "racial democracy" is a myth.
Most of the poorest Brazilians are Black.
Blacks comprise 70% of the poorest tenth of Brazilians and just 16% of the wealthiest tenth, the United Nations Development Programme said recently.
Afro-Brazilians earned an average of 173 Brazilian reals ($74) a month in 2000, less than half the pay for Whites in 1980, it said.
"The data merely corroborate what is already visible to any observer: The farther one goes up along the power hierarchy, the Whiter Brazilian society becomes," the UN report said.
Rio officials want to bring Black history more in the open by creating a walking tour and putting the cemetery on tourism routes.
"We want to make an open-air museum, with a tour from the docks to the cemetery, with bilingual folders and a map showing where slaves were displayed and sold," Zambelli said.
"Africa contributed to the founding of the city."
But Guimaraes is skeptical the city will invest in the cemetery that her workers stumbled on.
Officials have done little to preserve the bones, she said, and rains washed away some of the exposed remains. Her neighbours resent that she told the city about the cemetery.
"I don't have anybody's support," she said.
"People ask me why I'm doing this, but the more I learn about how the Africans were abused and realise it's been forgotten, I swear they won't forget it here, not while I have the strength."