I know it could rain there. Since it’s a huge underground area with people and other heat sources, water vapor would rise, condensate on the cold rock ceiling and fall in water drops. Guess if it’s a really huge area there could even be some sort of cloud formation and as that, when the temperature’s right: snow.
SsspAh, it's snowing...BpffI just know you're worr-...The truth is...!Shff...Auaaa♡Lady Koishi hasn't been coming home recently, has she...The surface world is a lot of fun, and she's flitting around playing there, I'm sure.She'll come home soon enough.But it's been several months, hasn't it?Sis...See? Speak of the devil.Our baby has been born, Sis!!