The Cappadocia region, with its very distinct culture, is one of the attractive tourist sites of Turkey due to its spectacular and unique landforms and historical heritage. In this region, the structures carved into thick and soft tuffs have survived and kept their integrity for about 12-13 centuries. The Zelve Open Air Museum, consisting of three valleys, is one of the oldest antique semi-underground settlements in the region and is designated as a World Heritage Site in Turkey. Although it was closed to human habitation due to collapses, which occurred within some of the rock-hewn structures and resulted in three casualties in 1950s, it is open to tourists. Particularly in the first valley, there are a lot of rock-hewn openings of different sizes and several churches with some geo-engineering problems. This study aims to assess the geo-engineering characteristics of the soft tuffs surrounding the rock-hewn historical structures and environmental conditions in the first valley of the Zelve Open Air Museum and their possible effects on the geo-engineering problems observed in this museum. The results mainly suggest that the strength of rock is reduced drastically under saturated conditions, and the processes of freezing-thawing and wetting-drying accelerate further degradation of rock under saturated conditions. Over a period of about five years, the amount of erosion, which mainly resulted from the two strong agents of rainfall and wind, ranged between 1.07 and 6.21 mm. Assuming that the erosion is homogeneous, determined annual average erosion ranges between 0.21 and 1.24 mm/yr. These observations suggest that erosion is an important phenomenon in this site and increases particularly in the winter and spring seasons. The failure modes, such as falling, sliding, bending, spalling, and slabbing, observed in the valley are considered to be mainly associated with deterioration and erosion of tuffs near vertical and laterally persistent discontinuities generally oriented perpendicular and parallel to the valley, and property changes due to cyclic freezing-thawing and wetting-drying processes. Spalling and slabbing occurring in cliffs and semi-underground openings are the indicators of yielding of rock. These phenomena result in the formation of slabs with different thicknesses, and consequently cause a change in geometry of the openings in the valley to trigger failures.