Let's say this at the outset: any sport can kill you.
When you push your body, ignore your limits, don't know your limits or let the adrenaline think for you, you're more likely than the average person to put yourself in harm's way. Every year football players die of heatstroke while in training; skiers run into trees; marathoners go into cardiac arrest.
But, no, we’re not talking about the inherent risks of being active and pushing yourself within everyday limits. We're talking sports where one mistake can mean a broken neck, pulmonary embolism or drowning.
Take freediving, for example: Competitive freedivers descend to depths of 400 feet or more on a single breath; open your mouth at that depth and over 170 pounds per square inch of water will rush into the breach and crush you from the inside.
Or free solo climbing: slip on a slick foothold and plummet thousands of feet to the valley floor. Or wingsuit flying, where factors out of your control, like a gust of wind, can send you into the side of a cliff at 200 miles per hour.
Even sports that one wouldn't normally put on the "extreme" end of the spectrum have their death-defying moments. Bicycle road racing can involve traveling down mountains at almost freeway speeds in tight formation. Go wide on a bend and it could be lights out—if not for you then your neighbor. (This happened in 2006 at the Six Days of Ghent race, when cyclist Dimitri De Fauw knocked Isaac Gálvez into a guard railing. Gálvez died of internal bleeding.)
And, of course, there's one activity that's the granddaddy of all extreme sports, having killed dozens a year for well over a century—click here to see which one.