The unexplained "Julia" sound is described as:
Julia is a sound recorded on March 1, 1999 by the U.S. National Oceanic and Atmospheric Administration (NOAA). NOAA said the source of the sound was most likely a large iceberg that had run aground off Antarctica. It was sufficiently loud to be heard over the entire Equatorial Pacific Ocean autonomous hydrophone array. The unidentified sound lasted for about 15 seconds. Due to the uncertainty of the arrival azimuth, the point of origin could be between Bransfield Straits and Cape Adare.
If you Google for information on this you invariably find a whole slew of web-sites that claim something along the lines of
The photo and all the information regarding this is all hush-hush and classified, but often explained away as underwater volcanic activity or a large iceberg scratching against the ocean floor. But pictures rumored to have been taken by Nasa’s Apollo AA35 suggest something more sinister.
Is there any evidence to suggest that anything about the Julia sound or its cause is classified?
In the same vein, how could these web-sites and others could have determined that any classified evidence exists? (E.g. are there any reports with redactions?)