In the archaeological site of Ancient Nemea, Greece, southeast of the Temple of Zeus, there is an ancient tunnel that was buried with earth until it was discovered in 1978. At present, some limestone blocks of the tunnel show appreciable damage mainly due to the humidity fluctuation within the tunnel. In this paper we present a comprehensive structural analysis of the underground tunnel based on the thrust line limit-analysis of the masonry arch and its surrounding soil. The study concludes that the tunnel with its overburden-surrounding soil is structurally stable and has ample bearing capacity. At the same time the study shows that the stones which suffer the most noticeable exfoliation due to the humidity fluctuation are those that their visible surface from the inside of the tunnel is in compression. In conclusion, selective stones of the structure need to be retrofitted in order to avoid further local failures, while the humidity fluctuation inside the tunnel needs to be minimized.