Cyrodil Tanks

Hawco10

I've been running around in a Cyirodil and I've noticed these virtually indestructible tanks roaming around. Do they have some special armor ? Or skills? Notice they are always cp600. Honestly, you should see one of them, they take a beating and then hop on off into the sunset.
So my question is, if I or anyone else wanted to be one of these tanks, what sort of stuff would I need ?