Final answer:
The context of 'Rhea' is unclear, but historically, climate change, disease, resource exploitation, and warfare are common causes for the collapse of civilizations, which may apply to Rhea too.
Step-by-step explanation:
Without a specific context of 'Rhea' being a historical world or city mentioned in historical texts, we do not have enough information to definitively answer what happened to the world named Rhea. However, looking at historical patterns of civilization collapses, various factors such as climate change, disease, environmental degradation, resource exploitation, and warfare often play significant roles in the decline of civilizations. These challenging conditions can lead to societies being left vulnerable to external pressures, including invasions or natural disasters, ultimately causing their downfall.
Considering our historical knowledge and the pattern that has emerged from past civilizations' collapses, the answer could potentially be a combination of those factors, much like the decline of the Indus valley civilization, Mycenaean Greece, and Easter Island's Rapa Nui culture.