The Wall Of Darkness by Arthur C. Clarke - Ages ago the First Dynasty had walled up half the planet, and left no hint of their terrible reason... Only Shervane dared to surmount that mile-high barrier, and learn the mind-shattering truth that lay beyond.
Many and strange are the universes that drift like bubbles in the foam upon the river of time. Some–a very few–move against or athwart its current; and fewer still are those that lie forever beyond its reach, knowing nothing of the future or the past.