Research into the Storegga tsunami that hit the coasts of Western Norway, Scotland, and beyond 8200 years ago has, until now, primarily identified its direct physical impact. However, the story of the Storegga tsunami is also about human–environment relations, capacities, and social strategies for risk reduction in Mesolithic coastal societies. This chapter is an attempt to broaden its epistemology, investigating the tsunami’s social impact through exploring the ontology of tsunamis and tsunami encounters via monsters. A monster/tsunami is an entity one may see and encounter; it can have agency and influence, representing risks and dangers that people or societies respond to or develop strategies for handling. This opens the range of what can be considered the materiality of the Storegga tsunami. Furthermore, building also on geomythology and anthropological disaster research, I recognize the power of storytelling and knowledge transmission that also enable explorations of other categories in the archaeological record rarely highlighted when the stories of the Storegga tsunami are written.