Perhaps not quite so established or as classical as the other offerings on this thread, but this one by Tolkein makes me think of the eternal mystery of ancient trackways. Nice to recite in the mind while walking over solitary high downs:

Down from the door where it began.
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.