What's lost+found and where did it come from?

I reformatted a hard drive to ext4, planning to use it as a backup drive. After mounting the freshly-formatted drive, I discovered a single empty directory inside it: lost+found. What's the purpose of this mysterious directory?

Solution 1:

lost+found is the directory in which fsck (filesystem check) will put files it restores from orphaned blocks. This can happen when something corrupts filesystem meta-blocks (also called i-nodes) in which the references of the blocks are stored which contain the data of a file.

Look also at http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/lostfound.html http://ubuntuforums.org/archive/index.php/t-229143.html