The Archéoforum is an archaeological site beneath the Place Saint-Lambert, accessed by a staircase from the square above. When construction work began on the square in the 1980s, excavations revealed continuous human settlement going back 9,000 years: Mesolithic hearths, Roman foundations, the ruins of Saint Lambert's Cathedral (demolished during the French Revolution), and medieval cemeteries.
The site is now a protected underground museum where visitors can walk among the excavated layers and see the accumulated history of the city's most important spot. The contrast between the modern paving above and the thousand-year cathedral ruins below is striking. A good audio guide puts the different layers in context. Entry is ticketed and the visit takes about an hour.