1997 - "Bloop"
http://en.wikipedia.org/wiki/Bloop

The Bloop is the most notable of a series of unexplained sounds recorded NOAA SOSUS deep sea microphone array in the pacific ocean during the 1990s. While the sound itself may seem beguiling, the range in which it was recorded over thousands of miles makes it the loudest noise ever recorded in Earth's oceans, estimated seven times louder than the song of a Blue Whale. While conventional explanations suggest it's the sound of Ice calving off of glaciers later the sound was described as being most likely organic in nature suggesting it being of animal origin due to the nature of variations and vibrations in the sound.

Other unexplained sound phenomenon believed to be associated with the "Bloop"

1997 - "Slow Down"
http://en.wikipedia.org/wiki/Slow_Down_(unidentified_sound)

1991 - "Upsweep"
http://en.wikipedia.org/wiki/Upsweep_(unidentified_sound)

1997 - "Train"
http://en.wikipedia.org/wiki/Train_(unidentified_sound)

1997 - "Whistle"
http://en.wikipedia.org/wiki/Whistle_(unidentified_sound)

1999 - "Julia"
http://en.wikipedia.org/wiki/Julia_(unidentified_sound)

A similarly related phenomenon is the 1960-80s Russian phenomenon of "Quackers" mysterious frog-like sounds that would seemingly follow Russian Submarines in the North Atlantic Ocean, seemingly moving along with and actively avoiding sonar pulses. The subs would encounter this strange phenomenon upon passing certain areas in the ocean on their patrols staying with them for prolonged periods of time before departing with a final "Quack" as the Sub leaves the patrol area. The sounds have been measured at 200kh/m, much higher than any sound produced by a man-made vessel.

http://hauntingeurope.com/2011/03/the-quackers-and-the-bloop/