Abstract In June 1992, five large rock caverns were unearthed in a village near Longyou County in Zhejiang Province, south China. The caverns were manually excavated about 2000 years ago in argillaceous siltstone of Cretaceous age. Faults are not well developed, however there are clay interlayers within argillaceous siltstone bedding at each cavern. Field investigations suggest that the ancients had realized the influence of the clay interlayers on the stability of caverns and altered their location and layout accordingly. Several preserved trial adits at the site are good evidence of this conclusion. These adits are apparently abandoned due to the presence of clay interlayers. This is probably the earliest known use of geological exploration by adit methods, an approach now widely used. In this paper, the engineering geological conditions, especially the development of the clay interlayers, are presented in some detail. Statistical analysis shows that the numbers and average thickness of clay interlayers in the five completed caverns are less than those in other outcrops. It is concluded that trial adit methods and experience by geological observation were adopted by the ancients 2000 years ago in the excavation of underground rock caverns.