*Clear throat* BELIEVE! IT'S SPELLED BELIEVE!

Anyway, If the universe is so immense (and that is "immense" by the way, not the way you spell it,) then what are the odds of said "spacemen" happening upon us?

And you're probably (that's right, PROBABLY, not prolly,) going to get a barrage of responses affirming that the Mayans were, indeed, killed by smallpox.

And you can build a lot if you have a few hundred years, a little ingenuity, and a giant pool of (slaves or off-season farmers, depending on your view,) to build it with.