The existence of moated mounds in the archaeological record of north-east Thailand has long been known, the majority constructed during the earlier first millennium AD. Despite considerable research, the purpose of the substantial and sometimes multiple moats surrounding raised occupation mounds has remained a mystery. Combining locational, hydrological and rainfall data with the archaeological evidence, this study of the moated mounds of the Khorat Plateau seeks to resolve the question through statistical analysis. The results suggest that water storage may have been the primary purpose of the moats, enabling communities to survive dry seasons and droughts.