The Nanda Parivrajya Sutra

"Suppose this world were one great ocean which dwells a blind, long-lived tortoise who surfaces once every one hundred years. Upon that ocean is a yoke with a single opening which is driven from east to west by the eastern wind, and so on. There will come a time when the outstretched neck of that blind tortoise enters the hole of that yoke. But I tell you, human life is even more difficult to obtain then that."