Presence of a radioactive gas in archaeological excavations, determination and mitigation

•High radon concentration, in a tunnel beneath the Feathered Serpent Temple at the archaeological site in Teotihuacan.•Dose determination and ssuccessful radon mitigation in this recent archaeological excavation.•Bring back confidence to archaeologists for working in a low natural radiation environment.

During recent archaeological discovery and excavation of a tunnel build approximately 2000 years ago by the Teotihuacans under the feathered serpent temple, in Mexico, abnormal radon concentrations up to 700 Bq m−3 were measured at several excavation stages. The tunnel is at 15 m below the earth surface with a vertical rectangular entrance of 4×4 m2, a semi cylindrical shape of 3 m in diameter, with a probable length of 100 m.This study supports the assumption that at the opening of the tunnel radon concentration was around 5500 Bq m−3; however, although natural convection in the tunnel atmosphere naturally pups radon out, it build up to a maximum concentration of 2000 Bq m−3. This paper presents the identification of the radon problem in this archaeological site, dose determination, and the mitigation actions that reduced the radon concentration down to 40 Bq m−3 that ensure a negligible radon risk for archaeologist.