The Tigris has been the lifeblood of ancient Mesopotamia and modern Iraq, but geopolitics and climate change have left it at risk of becoming uninhabitable. Writer, broadcaster and explorer Leon McCarron shares stories from his incredible, beautiful and occasionally dangerous journey by boat along the full length of the river, through the birthplace of civilisation in Turkey, Syria and Iraq. Sometimes harassed by militias and relying on the generosity of a network of strangers to reach the Persian Gulf, he explains why it’s crucial to save this extraordinary river, and what its survival, or destruction, could mean for us all