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By Jason Bittel
In the late 1990s, scientists discovered that elephants had a secret way of communicating, a vocalization so low in frequency it is imperceptible to the human ear. It's called infrasound. The ponderous pachyderms transmit these secret messages at least partly through the ground. When an elephant really lets loose, its infrasound can reverberate almost four miles through the rocks and sands of the savanna.