Construction workers in Bolivia accidentally discovered hundreds of human skeletons. They stumbled upon what appears to be a mass grave from the Spanish colonial era when they started construction in Potosi, a historic mining city. The skeletons are believed to be the remains of indigenous workers who toiled in the mines centuries ago. One hypothesis is that this mass grave is where slaves and indentured servants were buried. Another is that the grave was specifically needed after a reservoir break killed over 2,000 in the 17th Century. The modern-day construction workers claim to have found the remains of 400 to 500 people. "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 connected to Tomas Frias University in Potosi.