Remember pressures. Any fiery, earth-destroying demon/alien would consider where we live to be incredibly inhospitable, on the grounds that if they actually come up here, they will explode. And not even nuclear-style "wipe out all life on Earth, ha ha ha!" type explosion, but just a dead-fiery-earth-destroying-demon/alien-making-a-short-lived-unpleasant-mess sort of explode. You wouldn't want to be standing right next to it, but on the grand earth-destroying scale it'd be barely a blip.

Following this line of funny ideas, I'm betting that some followers of the Hollow Earth Theory[1] must be thinking:

"bah, more money spent to perpetuate the fiasco, why don't they just enter through the poles?"

And yes, there are still quite a few people believing this.[2]

[1] http://en.wikipedia.org/wiki/Hollow_Earth

[2] http://www.google.com/search?q=hollow+earth+conspiracy,

http://www.atlanteanconspiracy.com/2011/01/hollow-earth-evid...

The drill team will start every morning with a rousing cheer "Release the Kracken!"

I would expect it to be Cthulhu, myself. So, maybe not fiery, but earth-destroying demon/alien fits.

https://en.wikipedia.org/wiki/Cthulhu

You will like this Doctor Who episode if you didn't see it yet:

http://en.wikipedia.org/wiki/Inferno_(Doctor_Who)

The first thing that came to mind was this one: http://tvtropes.org/pmwiki/pmwiki.php/Recap/DoctorWhoS31E08T...

I thought of the Racnoss (from the Runaway Bride episode).

That is the Tarn Adams theory.