The Dark Thing

Something's thumping in my ears like thunder,
but it couldn't be my heart.
My heart's a million miles down
where soul and body part.
I might toss in a line to see what I can catch,
but this thing sounds dark and toothy
when it growls like a wretch.
At night it throws itself against my chest
and peers out through green eyes.
I think I'll let it be for now,
though it's eating me alive.