Back in the summer of 1997 NOAA repeatedly recorded a mysterious sound coming from about 50° South and 100° West. The sound was heard by various recording equipment thousands of miles apart and appeared to be organic. However, the strength of the sound would make it the by far the largest animal in the world.
The microphone system that NOAA used is known as the Sound Surveillance System or SOSUS. It was first designed to monitor Soviet submarines around the world. It works by listening into the deep sound channel, which is a level of the ocean where sound can travel thousands of miles without being dispersed or distorted.
The exact nature of Bloop is a mystery. Some scientists will suggest things like really giant squid or even really big ice sheet breaking. However, the fact nothing like it has been heard since 1997 is mind boggling. An probably unrelated sound, Slow Down, was heard 2,500 miles away at 15° South and 115° West.
The site Bloop Watch links to a NOAA page where one can compare Bloop and Slow Down to other known sounds.
While some geographers and nay sayers may complain that the world has already been explored and that there are no more "white spots" left on the map, just remember that there are plenty of blue spaces that need filling in.