A Visitor to a Museum

In a post-apocalyptic world (from an unspecified ecological disaster), the population is divided and decimated, including the so-called "degenerates" (mutants deformed as a result of some catastrophic event), and the survivors of the previous civilization. In the depths of the sea exists "the Museum"; it is a place which carries the remains of past civilizations and can only be reached during occasional periods of low tide when the sea becomes a barren desert.