The Halo franchise is about committing space genocide to dramatic music. At all times you're either shooting your guns, running to the next location for gun shooting, or waiting to respawn because you got shot too much. Naturally, some people looked at the franchise and said, "Hey, we should make this into a racing game!"
It's not entirely illogical -- Halo does feature vehicular sections, and also halos are round, like race tracks. And I think we can all agree that NASCAR would have a lot more widespread appeal if a second person rode on the back of every car and shot rockets at their opponents. But it's one thing to take a multiplayer map designed for firefights and scatter a few racing checkpoints around. It's quite another to spend hours in the game's level editor making custom racetracks that rival anything you see in actual racing games.
That course has more twist and turns than a spy movie, and fewer rails and safety measures than ... the same spy movie, I guess. Man, I'm terrible at metaphors today. My point is that I've finished most Halo games on the difficulty level where bullets are actually a realistic threat, and I'm pretty sure I'd fall off that course faster than I'd fall off a tightrope made of razor blades. And that's not even the hardest one!
Look at that thing, it's like an F-Zero track made love to a roller coaster and made space marines do rad jumps across their child's body. Videos of gamers completing courses are pretty damn impressive considering that the vehicles in Halo are designed to get players from Firefight A to Firefight B ... barely. If you don't have at least 20 hours of playtime, you'll definitely find yourself spinning out grill-first into the side of a cliff until your friend grabs your handle and says through gritted teeth, "Just fucking let me do it. Goddamn."
These gamers decided to get better at an ersatz racing game buried inside a shooter than most of us will ever be at actual racing games. I don't know if they're hugely dedicated Halo fans or if traditional racing titles just don't feature enough landmine dodging for their tastes, but either way, they're almost unfairly good at what they do. At least if you lose to them you can immediately finish first in the "not getting murdered by the sore loser's assault rifle" game.
You can read more from Mark, including how he made a cooking game out of Pong, at his website.
Head over to the forums to see other awesome games you can play inside games.. And also check out 5 Improved Versions of Classic Games That Fans Made for Free and The 7 Creepiest Hacks of Popular Video Games.