In the ever-shrinking world, the tourism industry is looking for new spaces to explore. It is redefining the known ones and reaching for new ones, which are often located right on geographical peripheries. While it may seem that only space travel remains a challenge for tourism today, terrestrial environments of the deep ocean and deep caves are still beyond the reach of not only mass tourism, but tourism in general. The goal of the article is to identify objective and subjective, stable or variable, external and internal conditions that explain why the deep ocean and caves represent the ultimate frontiers for tourism. The author questions to what extent this situation will continue in the future and discusses the attractiveness and development prospects of various niche forms of tourism based on the features of the underworld and deep waters which, at first sight, constitute barriers or threats to most tourism activities.