Rapid population growth occurred within Teotihuacán during the Tzacualli and Miccaotli phase. Many believe this is due to the relocation of settlements within the Basin of Mexico following volcanic activity, which impacted southern portions of the Basin. With this population being reorganized, vast amounts of construction efforts began to shape the ceremonial core of the city. Many have hypothesized that the Street of the Dead was established during this time, replacing earlier structures that now underlie this area, and large-scale construction occurred at the Sun Pyramid, the Moon Pyramid, and the Ciudadela complex.