A network of tunnels carved into the soft limestone beneath the upper town, accessed from the basement of the former Hôtel-Dieu (the medieval hospital) on Allée des Marronniers. The known network covers about 20 kilometres, though most of it is closed to visitors; the public tour covers a small but representative section.
The original purpose of the tunnels is still debated. The mainstream view is storage — Provins sits above a layer of soft chalk that holds a constant cool temperature, ideal for stockpiling goods during the trade fairs and protecting them from fire. Other tunnels were used for mining a thick deposit of fuller's earth that was essential to the medieval woollen trade. Walls throughout are densely covered with medieval graffiti — merchants' marks, masonic symbols, religious figures, and a tantalising mix of half-legible inscriptions.
Guided tours run multiple times a day in French (with printed English explanations available) and last about 45 minutes. The walk includes some narrow passages and steep stone steps.