Yellowstone National Park is home to a vast array of landscapes and a huge diversity of animals, united in a fascinating ecosystem, one that is currently in severe trouble. The area once contained over 19,000 elk, but its numbers have plummeted by 80% in the last two decades. The mysterious decline has sparked many questions, and researchers are in a race to provide answers. Join them as they investigate a series of theories and suspects, from wolves to bears to trees to trout, in an attempt to solve this alarming puzzle.

• Smithsonian Channel
• John Beach