How to download and install Darwin 13 OS in x86?

I've conducted a research on UNIX operating systems and found out that an experimental OS (Darwin) is used for development of the OSX; just like Fedora on Red Hat Enterprise Linux.
Darwin OS has a license of "Apple Public Source License" or freeware.
Darwin OS can be installed on x86 computer architectures according to Wikipedia.

I've searched all over the net and torrent sites but it seems that the links to recent versions of the OS like 13, 12, etc. are very hard to find.
I've tried the site "opensource.apple.com" but the "Download" section could not be found. I think that the links are hidden.

1) Where can I download a recent version of Darwin like 12, 11, or 10? I just found Darwin 8.0.1 here.
2) Can I download the source code and just compile it to ISO format?
3) Can you give me direct links?
Answers will help me a lot for studying Objective-C (Java programmer here).
Thanks!

The PureDarwin project is attempting to maintain a complete and bootable implementation of the Darwin OS. From their website:

One current goal of this project is to provide a useful bootable ISO of Darwin 10.x and Darwin 9.x.

Darwin itself is a modified FreeBSD OS with Mach microkernel.

Note though that having Darwin is not a necessary part of developing in Obj-C. Both gcc and clang support Obj-C development in newer versions. You are, of course, limited in what you can do relative to using Obj-C on an OS X machine because there are far fewer standard libraries available outside of the OS X world for Obj-C.