Everything is generated from apeiron, and must return there according to necessity.[18] A conception of the nature of the world was that the earth below its surface stretches down indefinitely and has its roots on or above Tartarus, the lower part of the underworld.[19] In a phrase of Xenophanes, "The upper limit of the earth borders on air, near our feet. The lower limit reaches down to the "apeiron" (i.e. the unlimited)."[19] The sources and limits of the earth, the sea, the sky, Tartarus, and all things are located in a great windy-gap, which seems to be infinite, and is a later specification of "chaos".[19][20]