User blog comment:Sebastian.F/404/@comment-2246689-20111110015917

The 404 seed is a Minecraft seed which has a gravel pit, which if you dig a part of gravel, it opens up a huge cave, but only works on versions that are 1.7 and before. You can see it here; http://www.youtube.com/watch?v=-L_YNRlRbDQ