A team of mining archaeologists has investigated a 5,000-year-old silver mine in Thorikos, Greece.

The cramped mines were likely to have been worked by slaves, who endured the lack of light, fresh air, and temperatures that hovered around 70 degrees Fahrenheit, as archaeology.org reports.

The team of mining archaeologists was supervised by Prof. Dr Denis Morin of the University of Lorraine, connected with the UMR CNRS 5608 (UMR National Center for Scientific Research 5608) of Toulouse.

The scientists employed a drone to locate above-ground installations connected to the mining. It is the first time that such complex mining infrastructure is studied.

“The progress of the underground survey required a constant vigilance in this stuffy space where the rate of oxygen must be permanently watched,” Denis Morin of the University of Lorraine said in a press release.

The mines are at the foot of the Thorikos Acropolis, which overlooks the harbor of Lavrio southeast of Athens, and consist of a network of chambers, shafts and galleries and many of the subterranean network’s ceilings are no higher than 30 centimeters.

The team members have also found tool marks on the walls of the subterranean galleries, graffiti, pottery, oil lamps, stone hammers, and crushing areas.

By the fifth and fourth centuries B.C., silver was extracted on a large scale with a sophisticated system from shafts cut through the rock.