Precipitous mountain roads, specially modified bikes, and deadly consequences. Simon Maybin spends time with the young men who race down the steep roads of Colombia’s second city Medellin. Marlon is 16 and he’s a gravitoso - a gravity biker. He hooks onto the back of lorries or buses climbing the precipitous roads to reach high points around the city. Then, he lets gravity do its thing and - without any safety gear - hurtles back down the roads, trying to dodge the traffic. This year, two of his friends have died gravity biking and Marlon has had a near-fatal accident. But he’s not quitting. So what drives young men like him to take their lives into their own hands? And what’s being done to stop more deaths? Produced and presented by Simon Maybin.