What are our bodies really capable of? We like to sit in air-conditioned comfort, yet each year millions of ordinary people train in CrossFit boxes, compete in Tough Mudders, and challenge themselves in Spartan races. They are connecting with their environment and, whether they realise it or not, unlocking their hidden evolutionary potential. No one exemplifies this better than Wim Hof, whose remarkable ability to control his body temperature in extreme cold has sparked a whirlwind of scientific study. Through him, we are just beginning to understand how cold adaptation might combat autoimmune diseases and chronic pain - and possibly even reverse the development of one of our greatest killers: diabetes. Award-winning journalist Scott Carney investigates the astonishing and sometimes dangerous world of body transformation. He reveals techniques you can try at home, but his own journey culminates in a record-bending, 28-hour climb to the snowy peak of Mt Kilimanjaro - wearing nothing but a pair of shorts and running shoes.