In January 1941, a section of the canyon wall known as "Threatening Rock", or "tse biyaa anii'ahi" (leaning rock gap) in Navajo, collapsed as a result of a rock fall, destroying some of the structure's rear wall and a number of rooms. The builders of Pueblo Bonito appear to have been well aware of this threat, but chose to build beneath the fractured stone anyway. The wall stood high and weighed approximately 30,000 tons; the Puebloans compensated by building structural reinforcements for the slab.