This interdisciplinary article argues that ancient Greek and Roman representations of Okeanos, Tartaros, and the underworld demonstrate an observational awareness of the hollow underground spaces that characterize the geomorphology of karst terrains in the Mediterranean world. We review the scientific facts that underlie Greek and Roman accounts of karstic terrain in observation-based discourse and in myths, and we demonstrate that the Greek words barathron (pit), limnē (lake), koilos (hollow), and dinē (whirling current) are used with precision in observational accounts of karst terrain. Ancient accounts of the dynamic limnē and barathrum systems characteristic of much karst terrain offer a complex matrix of observation-based and belief-based discourse: at the edge of the barathrum, the imaginative or spiritual realm of the underworld has a material plausibility and the closely observed material world has a plausible potential to connect to a world we cannot see.