Lensfinder.com - Ultra Rare Nikkor 6mm f/5.6. Only a few hundred of these lenses exist. This lens has a 220 degree field of view for Full Frame 24x36mm sensors. Made in Japan
6mm f/2.8 Fisheye-Nikkor on a Nikon D800 - First Time Ever? - YouTube
Not for the frugal fisheye photographer: Rare Nikon 6mm f/2.8 Ai-S lens available on eBay for around $85k
This Uber-Rare Nikon 6mm f2.8 Fisheye Lens Can See Behind Itself - The Phoblographer
Ridiculous Nikon 6mm Lens Reappears, Priced at $160,000+ in the UK | Fish eye lens, Nikon lens, Nikon lenses
Camera Geekery: The Nikkor 6mm f/5.6 Fisheye Nikkor aka The Holy Grail - Japan Camera Hunter
The $50,000 Lens That Can See Behind Itself
An image taken with a 6mm f/2.8 Fisheye-Nikkor lens | Flickr