It’s distinctively biological in origin. Something is down there.
I’m picturing either…
A huge, tentacled, Old One, just about to surface and devastate the planet with its otherworldly wrath.
Or…
A giant unicellular organism, which spends its entire life anchored to the sea bottom, accumulating gases slowly over centuries, so that, just at the moment of its death, it can release all the gas at once, making an enormous “bloop!” sound and amusing its neighbors to no end.