You are standing at the edge of a well in the hottest city you have ever seen. The stone rim is worn smooth by four hundred years of hands. Gold dust fills the cracks. Drums are beating. A girl in white is walking toward the well, singing a melody of four notes that means "I am the rain."

The Soninke people of West Africa built an empire called Wagadu. The Arab geographers called it Ghana. It was the first great state of sub-Saharan Africa. Its wealth came from controlling the exchange point between Saharan salt and southern gold. Al-Bakri described a capital where dogs wore gold collars and horses were tethered with ropes of woven gold thread.

The mechanism that sustained this wealth, according to the Soninke oral tradition, was a pact with a serpent. The Bida lived beneath the central well. Seven heads. Older than the empire it fed. ...