Some observatories (including this one it seems) use liquid mercury as mirrors: https://sunspot.solar/about/ https://sunspot.solar/about/ https://en.wikipedia.

Some observatories (including this one it seems) use liquid mercury as mirrors: https://sunspot.solar/about/ https://sunspot.solar/about/ https://en.wikipedia.org/wiki/Liquid_mirror_telescope https://en.wikipedia.org/wiki/Liquid_mirror_telescope