Horizontally, the earth is conceived in various ways: as a square with its four directional or, perhaps, solstice points, or as a circle without such fixed points. The square earth is sometimes imagined as a maize field, the circular earth as a turtle floating on the waters. Each direction has its own tree, bird, deity, colour, and aspect, in the highlands also its own mountain. Vertically, the sky is divided into thirteen layers, and Classic period deities are sometimes linked to one of the thirteen skies. By analogy with the 'Nine-God' mentioned together with the 'Thirteen-God' in the Chilam Balam book of Chumayel, the underworld is often assumed to have consisted of nine layers. However, the Popol Vuh does not know such a ninefold division, and Classic period references to layers of the underworld have not been identified.