Archaeologists in the ancient Roman city of Pompeii they discovered a “prison bakery” where slaves and donkeys were locked inside to grind grain to make bread.

A narrow room with no view of the outside world and small windows high up in the iron-barred wall was uncovered during the excavations, which are being carried out as part of a wider project to shore up hillsides around unexplored areas of the city.

Indentations in the floor coordinated the movement of the animals, who were forced to move blindfolded for hours, the Archaeological Park said in a statement.

“It is a space in which we have to imagine the presence of human slaves… it is the most shocking side of slavery in the ancient world” and its “brutal violence,” said Gabriel Zuchtriegel, director of the Archaeological Park of Pompeii.

The bakery was part of the working space of a large house whose living space was decorated with well-preserved frescoes.

Archaeologists in August discovered a small bedroom in a Roman villa near Pompeii that is believed to have been used by slaves, shedding light on the conditions in which they lived in the ancient world.

Pompeii and the surrounding area were submerged in volcanic ash when Mount Vesuvius erupted in AD 79, killing thousands of Romans who were unaware they were living beneath one of Europe’s largest volcanoes.

The explosion buried the city in a thick layer of ash, preserving the form of many of the city’s residents and buildings.

The ancient city has seen a flurry of archaeological activity in recent years aimed at reversing years of neglect, largely thanks to a recently completed project funded by the European Union to the tune of €105 million.