For conservation biologists, determining the onset of deleterious change through recognising baseline conditions is regarded as being critical for implementing the effective management and restoration of anthropogenically altered marine ecosystems. In particular, mining information contained within historical anecdotes from non-traditional sources can provide valuable insights about past environmental conditions. The present study demonstrates that careful parsing of eyewitness descriptions of unidentified marine objects (UMOs), considered at the time to have been sea serpents, reveals that the onset of African marine fauna becoming entangled in fishing gear or maritime debris predates, by more than a century, the advent and widespread use of plastic. This work joins other environmental histories in challenging the misconception of a destructive modernity that can be easily differentiated from an exalted past. Such a reinterpretation of what were imagined to be sea serpents joins a new re-evaluation of Mary Shelley's Creature, both serving as metaphors of monstrosity -one, concerning the walking, the other, the swimming, undead -for social-ecological upheaval during the Anthropocene.