MADISON, WISCONSIN—A massive flood 1,000 years ago may be responsible for the decline and eventual abandonment of the prehistoric city now known as Cahokia. Samuel E. Munoz of the University of Wisconsin examined cores from nearby Horseshoe Lake and found a thick layer of sediment followed by a decline in pollen from corn cultivation. The high waters probably did not reach Monk’s Mound, at the center of the city, but it may have forced as many as 15,000 people away from residential and agricultural areas. “When we realized we were looking at a flood, and that it fell right at this key time in Cahokia’s history, it was very exciting,” Munoz said.