If you will indulge me for a second, I’ll explain a bit further.
Some glasses (Magnesium fluoride, Calcium fluoride, fused silica) are transparent in what we call vacuum ultraviolet wavelengths (~120-300 nm, where visible light is 400-700 nm).
The region from 10-100 nm is known as extreme ultraviolet (EUV), where nothing transmits, and few materials even reflect with optical quality.
Once you get to shorter wavelengths, you get into the x-ray region, where materials again become transparent. So anything that is transparent at all in the EUV is a big discovery.
MD, who builds EUV cameras for a living