Ancient Egypt was a civilization of ancient Northeast Africa. It was concentrated along the lower reaches of the Nile River, situated within the contemporary territory of modern-day Egypt. Ancient Egyptian civilization followed prehistoric Egypt and coalesced around 3100 BC with the political unification of Upper and Lower Egypt under pharaoh or king Menes. The history of ancient Egypt unfolded as a series of stable kingdoms interspersed by periods of relative instability known as "Intermediate Periods". The various kingdoms fall into one of three categories: the Old Kingdom of the Early Bronze Age, the Middle Kingdom of the Middle Bronze Age, or the New Kingdom of the Late Bronze Age.