The often-reliable Wikipedia has, among its 46 articles not about cartoons, a list of unexplained sounds.
The bloop is a peculiar noise picked up by undersea microphones in the south pacific in 1997 (not far from R’lyeh, as any Lovecraft fan will know) and “while the audio profile of the bloop does resemble that of a living creature, the source is a mystery both because it is different from known sounds and because it was far too loud: it was several times louder than the loudest known biological sound.”
Julia (ocean child?) is another submarine sound, which apparently sounds like something very big and very far away saying “julia.” (note that this recording is sped up 16 times)
Only the vaguest ideas have been put forward to explain these sounds, from calving ice sheets to some huge, undiscovered creature. As a fan of huge, undiscovered creatures I’m going with that one.