The world’s biggest snake resides in the Amazon, and with the largest predators being one of the best indicators of the condition of an environment, a single scale from the giant green anaconda has the ability to reveal the health of the entire ecosystem. What is a dream and thrilling prospect for Professor Bryan Fry is a daunting reality for @WillSmith. Watch Pole to Pole with Will Smith, Wednesdays at 8pm on National Geographic UK Also streaming on Disney | National Geographic UK