For the last 25 years scientists have been left baffled as to the cause of mysterious ocean noises that originate from the depths of the Pacific Ocean. The strange sounds, known as the ‘Upsweep’, were first discovered in 1991. Scientists initially believed the sounds to be whale songs or ship vibrations, but these theories were quickly ruled out after further investigation. Express.co.uk reports: The National Oceanic and Atmospheric Administration (NOAA) has described Upsweep as “a long train of narrow-band upsweeping sounds of several seconds duration each”. The Upsweep you can hear in this extract is played at 20 times the normal speed. The sound got louder until 1994 and has gradually subsided since, but can still be detected. The NOAA believes it peaks in the spring and autumn. Because of the large number of volcanoes in the Pacific, including submerged ones, volcanic activity is one suggested explanation. Emile Okal of Northwestern University in Chicago and Jacques Talandier, formerly with the French Atomic Energy Agency, hypothesised in 1996 it could be the sound of seawater contacting a large pool of hot lava. Christopher Fox, director of NOAA’s acoustic monitoring project, said the movement of seawater can make noises like the movement of [...]