Earth's oceans are a cacophony of often bizarre noise, caused by a variety of sources, ranging from volcanic tremors to ships, to aquatic mammals such as humpback whales [source: Bobbitt]. Scientist from the National Oceanic and Atmospheric Administration have been studying these sounds for years, with the help of an underwater listening network that uses hydrophones, which produce voltage signals over a range of frequencies as they pick up underwater sounds from any direction [source: Fox].
One mysterious unexplained sound is the Upsweep, a set of narrow-band upsweeping sounds, each lasting several seconds and first detected by scientists in 1991. The Upsweep seems to be seasonal, and usually reaches its peak in the spring and fall, though no one is sure why. The source level is high enough to be recorded throughout the Pacific [source: PMEL].