It was the roar of a howler monkey, one of the loudest cries of any animal on earth. The sound, which can be heard from three miles away, is formed when the monkey forces air through its large, hollow hyoid bone, which sits between its lower jaw and voice box and anchors its tongue. The result is a deep, resonating howl that vibrates through the forest with strange, inhuman intensity, and echoes so pervasively that its location can be nearly impossible to identify.
