Why is there a // directory?

I just found a very weird feature (bug?) with my computer's file system. I can do cd // and it will go to the // directory, but display all the same files as the / directory. Why is this? If I cd .. while in /, it will stay at /. // is the only one that works -- I tried multiple slashes, but it just stays in /.

From the POSIX spec:

3.266: ... Multiple successive slashes are considered to be the same as one slash.

4.11: ... A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.

The second part means that a path beginning with // can have a special meaning. This is rarely if ever used, and can be a source of bugs: https://stackoverflow.com/a/7816833/163956.

It seems like Bash will normalize pathnames, but does not normalize double slashes at the beginning of a pathname. This is understandable, as on some Unix systems (though not Mac OS X), // may indicate a network path and Bash is intended to be portable. See this question on Unix.SE for the double-slash issue.

Since in Mac OS X // has no special meaning, you're actually in /.