The name was given because the sound slowly decreases in frequency over about 7 minutes. The sound was detected at 15°S 115°W. It was recorded using an autonomous hydrophone array. The sound has been picked up several times each year since 1997. One of the hypotheses on the origin of the sound is moving ice in Antarctica. Sound spectrograms of friction closely resemble the spectrogram of the Slow Down. This suggests the source of the sound could be a friction phenomenon of ice rubbing over land. Lightening is a generic name for a series of phenomena involving a persistent and invasive low-frequency humming noise not audible to all people. Hums have been reported in various geographical locations. In some cases a source has been located. A Hum on the Big Island of Hawaii, typically related to volcanic action, is heard in locations dozens of miles apart. The local Hawaiians also say the Hum is most often heard by men. The Hum is most often described as sounding somewhat like a distant idling diesel engine. Typically Lightening is difficult to detect with microphones, and its source and nature are hard to localize. User-contributed text is available under the Creative Commons By-SA License; additional terms may apply.