Like your friend, I have had the intriguing experience of staring down into a 10m deep water pool containing used fuel elements from a research reactor. The blue glow is dramatically beautiful and totally unearthly.

10m of water is OK. 10m of air is suicidal.

For those unfamiliar with the phenomenon, Cherenkov radiation occurs when high mass particles are so energetic that they travel faster than the local speed of light in a dense medium. (The local speed of light is often significantly slower than C, the speed in vacuuo). These massive particles leave the optical equivalent of a sonic boom - a shock wave.

It's quite easy to generate Cherenkov radiation in water, since the local speed of light is about 25% slower than in vacuum (hint, think about refractive index, Snell's law, and the velocity implications of refraction). To do it in air requires a VERY intense radiation source.

Some of the stories of what people saw that night are almost incredible. However, they are generally second-hand accounts, since the original witnesses perished shortly thereafter. To misquote "There are some things man was never meant to see".