It is 10,000 years in the future. Archeologists know that there once existed, many centuries ago, an ancient country called the United States of America. All that is left of that once mighty nation is a barren wasteland, bordered by a large wall on each of its four sides.
Experts are pretty sure they know the purpose of the wall to the East. That was built to keep out the waters of a mighty ocean, as increasing global temperatures caused that ocean to rise higher year after year, threatening to flood the land and devastate the populace.
The wall to the West served a similar purpose. Like its cousin three thousand miles to the East, this wall was under continual construction. It needed to be made higher with each successive year, to keep out the ever rising ocean waters of another ocean. Eventually both walls failed, as global warming inexorably overwhelmed the limits of human engineering.
The meaning of the wall to the north is well understood, for its purpose was preserved in history books. That wall belonged to another ancient nation named Canada, to keep the fleeing American population from rushing its borders. The history books record that the Canadians built this wall and made America pay for it.
Finally, there is a fourth wall to the South. This one puzzles the Archeologists, for it doesn’t seem to have served any useful purpose at all.