Technically it's any medium. The lower the refractive index, the closer the particle needs to travel to the speed of light in vacuum. But you can for example measure Cherenkov Radiation in the air (where n~=1.0003 or 99.97% of c) from highly energetic cosmic rays.

https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_...

Not really equivalent because physics can model the difference between "in water" and vacuum quite well. Definitely far better than biologists understand mice and humans