404 Challenge happens on the seed "404"
there's a patch of gravel that if you dig on it, the gravel falls like a sinkhole and creates a tunnel to the bottom of the map where there's a huge cave system.
The point, is to collect items from the top world during the first day cycle, and when nighttime falls, you must dig the gravel and travel to the bottom of the map to survive.
Oh, and torches aren't allowed.
It's really a cool thing to try and it's muuuuuch harder and indepth than it seems.
Edit:
Here's the link to the official challenge rules/guidelines.
http://www.minecraftforum.net/topic/175468-ironman-challenge-series-404/