At the end of an exciting cross-country race, the first question most spectators are likely to ask is: what's the deal with all these Olympic athletes lying down on the ground? Cross-country skiing is an incredibly demanding event. It's the marathon of the Winter Games, an endurance sport that has skiers gliding, skating and sprinting their way across as many as 30 kilometres (sometimes with a rifle on their backs, as in the biathlon) before tumbling across the finish line. Already during the Sochi Games it's been the venue for at least one seriously uplifting (and patriotism-inducing) story. But the sport does have one unusual tradition, which is notable at any cross-country competition. At the finish line, cross-country skiers — no matter what position they finished in — tend to collapse and lie prone on the snow for a few minutes. Why? Basically, they can't do anything else. They've been working so hard, pushing themselves to such an extreme physical limit, that after the final sprint to the finish they can barely breathe, let alone stand up. Slate has a fascinating, in-depth explanation of the phenomenon on their website. According to Dr. Dan Heil, an exercise physiologist at Montana State University, "you combine an insatiable need to breathe as hard as possible with leg and arm muscles seizing from acid build up, along with a central nervous system unable to control the act of standing and gliding upright, and the result is skiers dropping to the ground because they cannot stand and they need to recover.” Above, we've got a gallery of pictures of athletes collapsed on the snow.