Construction workers in Bolivia have stumbled upon a mass grave with what are likely the remains of hundreds of indigenous miners who died during the Spanish colonial era, according to a researcher.

The workers found the remains recently as they started construction on a new building in the "El Minero" district of Potosi, a city in southwestern Bolivia located high up in the Andes.

"We are talking about a common grave found at about 1.8 meters (5.9 feet), and the human remains are scattered over an area of four by four meters," said Sergio Fidel, a researcher at a museum belonging to Tomas Frias University on Saturday.

In the Spanish colonial era, Potosi became famous for its massive silver and tin reserves, which were first mined in the 16th century.

Local indigenous people, mainly ethnic Aymara, were commonly put to work as both slaves and indentured servants, especially at the famed Cerro Rico, or Rich Hill. Named a UNESCO world heritage site in 1987, along with the city of Potosi, the mountain represents a history of tremendous wealth but also suffering for the indigenous people and enslaved Africans who died mining it, earning Cerro Rico the title “the Mountain That Eats Men.”

The construction workers, who made the discovery and who have had no specialized excavation training, say they found the remains of 400 to 500 people and that there may be many more.

The university got involved when its staff learned the workers were piling the bones in a massive heap, fully exposed as construction continued.

One hypothesis is that they happened on an indigenous burial ground of slaves and indentured servants who would have worked at the mine in precarious conditions, said Jose Antonio Fuertes, a historian at the national mint.

Another possibility is the remains could be linked to the collapse of a reservoir in Potosi during the 1600s, which killed some 2,000 people.

The Andean city, once among the world's biggest cities, now has a population of 200,000.

Last month, UNESCO placed Potosi and the increasingly unstable Cerro Rico on its World Heritage in Danger list due to "uncontrolled mining operations."

Al Jazeera with AFP