The sipapu/<shibapu>〈〉 is the small round hole in the floor of the kiva ruin. The larger round hole in the floor is a fire pit. Observe that the air intake (little rectangular door in the wall), the stones that block air from the air intake, the fire pit and the sipapu are all in a line; this aspect of the design was intentional. Photo taken at Long House, Mesa Verde National Park.
Sipapu is a Hopi word which refers to a small hole or indentation in the floor of a kiva. Kivas were used by the Ancestral Puebloans and continue to be used by modern-day Puebloans. The sipapu symbolizes the portal through which their ancient ancestors first emerged to enter the present world.
Moreover, The Hopi Sacred Stories explain that this is the hole in which the first peoples of this world entered. As "They" stepped outside of the "Sipapu", they morphed from lizard-like beings into homo sapiens, or human form (See Waters, 1963, and later reprints; Courlander, 1971). It is from this point that the "First Peoples" of the Earth began to divide and separate, creating differing tribes along the first journeys of the first humans.