Ok, here's my tuppence worth, as a latecomer and admitted massive Lovecraft fanboy.
Reasons for Cthulhu:
1) Proximity to R'Lyeh coordinates.
2) Soundfile gives me the willies, really.
3) We may never here this again, maybe the Great One was just turning in his sleep?
4) Interestingly, if this were mundane (tectonics etc) why report it?
5) If there's a possibility of it being an encroachment on the waters of a nation, would they likely admit to a breach?
6) With a posited (?) End Of Days scheduled for 2012, could it be the Great Cthulhu rousing from his slumbers?
7) If it is, can I have the day off work?
I'm feeling fairly ambivalent towards this, yet, it interests me more than a lot of other stuff on here, such as Ufology, which just seems to spew
endless yay/nay-saying. I think there's a distinct possibility it could be an undersea gas release and am going to draw a slightly... jovial
comparison:
If you look at the timeframe involved of the bloop, in the grand scheme of things it is still relatively short, whale sound tends to go on for longer
periods. If you think of sitting in a bath full of water and passing wind, think of how much deeper and more gutteral it sounds, especially when
compared with one let out in the open. If this is simply a large gas pocket under the surface being released, it could conceviably last for that sort
of time period, if not longer - a large enough gas pocket could also take a long time to release.
Now, if someone (hopefully) has a decent background in undersea geology, tectonics etc and could disprove me, I'll dust my robe off.
Ia!!!
Chthulu Fhtagn!!!

