Spearfishing off Cavo Greco,Cyprus can hear the Turkish ship "Barbaros" carrying out seismic surveys (illegally breaching the territorial waters of Cyprus). You can hear the first sound waves fired at 0:21 and subsequently every 10 seconds after that.
Definition of seismic surveying from wikipedia:
"Traditional marine seismic surveys are conducted using speciallyequipped vessels that tow one or more cables containing a series of hydrophones at constant intervals....The seismic source, usually an airgun but other sources are available, is also deployed underneath the surface of the water and is located between the vessel and the first receiver. 2 sources are often used in order to achieve a fast rate of shooting....and the source is typically fired every 15 or 20 seconds.
http://en.wikipedia.org/wiki/File:Dia...