In order to investigate the causes of underground collapse and aid the development of prevention strategies, we conducted a detailed study of the physico-mechanical behaviour and ageing of chalk in underground quarries in the Parisian Basin, France. Core samples were drilled horizontally from pillars at two sites: the Saint-Martin-le-Nœud underground dolomitic chalk quarry (Oise, France) and the Estreux underground glauconitic chalk quarry (Nord, France). Uniaxial tests were performed on dry and saturated cylindrical specimens, extracted perpendicularly to the cores at regular distances along their lengths. The cores were drilled horizontally from the edge to the centre of the pillars. Measurements of both physical and mechanical properties were made. The dolomitic chalk (Saint-Martin-le-Nœud) exhibits variations in mechanical and physical properties with horizontal pillar depth, and both density and compressive strength appear to increase outwards from the core to the wall of the pillar, the opposite of what would be expected from a purely mechanical point of view. Scanning electronic microscope (SEM) analysis revealed progressive degradation and increasing homogeneity of the grains from the edge to the inside of the pillar. At the pillar wall, the crystal faces are well-defined, almost euhedral, and the grains are of variable size. In contrast, crystals in the inner part of the pillar are finer-grained and anhedral with highly degraded edges. In addition, video logging of the pillar revealed a greater number of fractures in the centre of the pillar than at its edge. In physico-mechanical tests and SEM analysis performed on the Estreux glauconitic chalk, no significant variation was observed between the pillar wall and the pillar core. However, dissolution marks were observed along the entire length of the half-pillar core.