180 degree vision masks do exist, however the ones that i have come across do not have glass lenses.
Browsing on the net, i found the Seal from Aqualung.
Another is the Sphere from Technisub. I tried this one on at a dive shop. Awesome view. Very slight distortion, though. The shop owner told me that the distortion is too much for some people. The best part was the fit. Beats the Cressi Big Eyes hands down when it comes to panoramic vision.
[This message has been edited by Manta (edited 08-11-2001).]