'The Bloop' was a low frequency sound that was 'romantically' attributed by some people to a large, unidentified and unknown sea creature. (A creature that only talked once in the last 15 years!)
You can read more about it and other deep sea mystery sounds at this link, hear Brian Dunning's account of it on the Skeptoid podcast here, or hear a somewhat repetitive Youtube video of the event here.
But now we finally have an explanation for this sound which was loud enough to be heard 5000km from its source. (Note that sound travels anomalously well under water, but that is still pretty surprising.) It seems, according to an account of the event by NOAA that it is consistent with the sound of an 'icequake' - cracking of a glacial shelf. The sounds are propagated through the ocean in such a way that a bloop would be heard. They say:
The broad spectrum sounds recorded in the summer of 1997 are consistent with icequakes generated by large icebergs as they crack and fracture. NOAA hydrophones deployed in the Scotia Sea detected numerous icequakes with spectrograms very similar to “Bloop”. The icequakes were used to acoustically track iceberg A53a as it disintegrated near South Georgia Island in early 2008. Icequakes are of sufficient amplitude to be detected on multiple sensors at a range of over 5000 km. Based on the arrival azimuth, the iceberg(s) generating “Bloop” most likely were between Bransfield Straits and the Ross Sea, or possibly at Cape Adare, a well know source of cryogenic signals.
In some ways it is a shame to spoil a good mystery, but if I took that attitude with everything then I would never have any feeling of satisfaction at work.
Old mysteries are still fun, even if you have an explanation!
Small note: I'm not sure that icebergs count as cryogenics!