>> it's worth the danger because when you know what you're doing, it's not all that dangerous. from a collision in the air to one on the ground. a professional mountain bike race at the albert falls dam and game reserve in south africa quickly turns from a friendly contest -- >> five, four, three, two, one, go. >> into a life and death confrontation when buck meets biker and spoiler alert, the biker isn't going to fair well in this close encounter. >> holy cow! >> the victim of this terrifying impact is 17-year-old professional biker evan. it was only five years earlier that his father bought evan and his brother their first books for christmas. >> the first years of riding, a family thing together with my dad. and from there my brother and i started getting serious and racing.