Half of the available range is packed into tiny distance from the near plane to twice the near plane distance.
This quote from this article of the Outerra blog is more or less its punch line. The author explains the precision issues with typical depth buffer use, and explores ways to get better results. Since Outerra is a planet engine, no wonder depth precision must be critical.
I discovered Outerra and its blog fairly recently, through a couple of mentions in the Flipcode’s Daily Flip, and was impressed by its rendering. The amount of work that went into it must be insane. See these captures demonstrating the space to ground transition or the grass rendering for instance: