What is the bloop?

"Theories abound as to the source of the Bloop. If it is the vocalization of a living organism, it is one which makes its home in the dark, cold depths of the ocean. Some have suggested that giant squids could be responsible for the sound, but that is unlikely considering that no known species of cephalopod have the gas-filled sac necessary to reach such great volumes. Indeed science has not recorded any animals– living or extinct– with nearly enough size to house the organs needed to produce the level of output demonstrated by the Bloop… so unless this mystery creature uses some unknown mechanism to generate sound, it is presumed to be an incredibly massive organism."