Click on a date in the calendar above to listen to an archived recording of the previous 48 hours of Earth sound. The average (rms) energy recorded each day is represented by the color scale ; the brighter the color, the greater the energy. An isolated bright spot in the calendar usually indicates a large transient event on that day (a major earthquake). Several consecutive days of bright purple usually indicate an increase in the microseismic background, caused by the passage of ocean storms. Audio volumes have been normalized: in recordings containing low energy (dark colors) the audio volume is increased to highlight the Earth's natural background ambience; during energetic periods (bright colors) the volume is reduced to accommodate the powerful signals from large events. Audio is unavailable on days marked ×.