Found here, this is the world's rarest and most expensive fisheye lens.
Now that is a massive piece of glass!
First introduced in 1970 at the Photokina trade show in Cologne, Germany, the Fisheye-Nikkor 6mm f/2.8 lens offers an angle of view of 220º making it, at the time, the "world's most extreme wide-angle lens to cover an image area of 24x36mm".
Apparently, it recently sold for 160,000 dollars. Better not scratch it!