| At Xochicalco we find a number of subterranean galleries partially hollowed out by human hands. In two of them, vertical shafts extend from a cave to the surface of the ground above. An observer situated beneath one of these openings 12 meters below ground level can see a small circular patch of blue sky overhead. On the occasions when the sun passes the overhead point, it throws a powerful ray of light into the darkened environment. These shafts may have served as zenith solar observatories. |