Past the racks of hair accessories on the ground floor of the Monoprix supermarket on the corner of the Rue Réaumur and the Boulevard de Sébastopol in the Second Arrondissement, there is a door marked staff only.

Slip through that passageway and turn left down a spiraling metal staircase into the basement. Past pallets of juice and soda bottles, down another flight of stairs, you will find a grim reminder of Paris’s history: a mass grave, with row upon row of medieval skeletons, 316 in total.

Archaeologists believe the discovery, unearthed in January, is part of the cemetery of a medieval hospital called the Hôpital de la Trinité that used to stand nearby. The long-buried mass grave is a reminder that Paris, for all its surface grandeur, is still replete with undiscovered archaeological treasures, some grand, others much more grisly.

For archaeologists, though, grisly can be good.