Plug in your headphones and turn up the volume. We're celebrating Earth Day by enjoying one of our planet's disappearing resources: sounds of nature unadulterated by noise pollution. Acoustic ecologist Gordon Hempton has made this possible by embarking on a personal quest to capture and preserve Earth's rarest sounds—those unaffected by human development.

Hempton, who has circled the globe numerous times in his 30 years—recording everything from the big-city bustle to the natural quiet of remote regions—has been cataloging quiet places for years, those he defines as locations where he can record an entire tape without the interruption of man-made noise. In 2010, when filmmaker Nicholas Sherman followed him for a documentary called Soundtracker, Hempton said there were less than a dozen places left in the contiguous 48 states where we could experience true silence. In this clip from the movie, you can watch Hempton on his search for silence and experience some of the beautiful sounds he tracks down. You can listen to many of his recordings on iTunes and watch a new clip of Hempton in the South American jungle here.