A series of potential pitfalls (fallacies) in estimating subterranean biodiversity are outlined: (1) provincialism—treating different regions differently, especially with respect to new discoveries and undescribed species; (2) equality of described and undescribed species—ignoring the possibility that undescribed species are not really new species; (3) isotropy—assuming all cave regions of similar size have equally rich faunas; (4) scale invariance—ignoring the affect of area on species richness; and (5) misuse of expert opinion—the over-reliance on experts estimates often without comparable estimates for all areas. Some standard procedures are suggested for subterranean biodiversity studies, and the value of such studies is emphasized.