<p>This study presents a comprehensive structural stability assessment of the ancient Serapeum tunnels beneath Pompey’s Pillar, integrating site observations, geotechnical investigations, and PLAXIS numerical analysis. Geotechnical and petrographic data classify the foundational porous Calcarenitic limestone as “weak to very weak” (UCS ≈ 5–6 MPa; RQD 15–20%). The analysis identifies critical total normal and shear stresses concentrations, (<i>σ</i>₁ = −3160 kN/m²) and (Sig_xy or <i>Ʈ</i> = 1063 kN/m²) at the tunnels’ crown zones and corners, creating localized zones of plastic deformation with a low factor of safety. Also the large deformations of this calcarenitic rock create zones in a fully plastic state of stress. The findings conclude that while the structure is currently stable with factor of safety (FoS) = 1.55, its long-term integrity is vulnerable to the continuous degradation of this soft rock, particularly at the identified high-stress hotspots due to the impact of flash flooding due to climate change.</p>