Complex sentence: The remains of human civilization live on ramshackle floating communities known as atolls, having long forgotten about living on land, though believe there is a mythological "Dryland" somewhere in the world.
Split into simpler sentences: The remains of human civilization live on ramshackle floating communities known as atolls, having long forgotten about living on land. Even though, people still believe that there is a mythological "Dryland" somewhere in the world.