Okay, I got this idea from "The Hobbit". It's a riddle, and you have to guess what the poem's talking about. You can't read the other replies to get ideas. And I wouldn't mind reviews about the writing. I didn't post it just for people to try to and guess what it means.
This very thing is an unseen foe
That haunts the world with no escape.
It is not fought, for it cannot be vanquished
Yet day by it works for doom.
So slowly that it is not noticed
Until it conquers all too soon.
The minds of men it grasps within
Its cunning, ancient hands
Until it leaves a shell of what
Was once a learned man.
An indestructible fortress is built
That no army can but scratch
And horribly, but subtly
And with its strength unmatched,
It tears them down to rubble
In a miserable heap of stone.
Galaxies it shifts and changes
By the bidding of its whim.
Oceans it dehydrates
And countries are its altering limb.
Not one thing has outmatched it.
No weakness can be found.
But in its daunting, gradual way
It turns the universe around.
