Update: I don't know how, but this happened twice in my world after some more exploring. wow.
Terraria Community Forums
Pericles_Corde
Stardust Guardian
What is the seed?
Pericles_Corde
It's in the ModLoader. Not sure if seeds exist there. 
But if it does, I'm willing to share, I didn't use any world altering mods. As far as I know at least.
But if it does, I'm willing to share, I didn't use any world altering mods. As far as I know at least.
Stardust Guardian
go to the config.json file of your terraria game in my games folder and find "Useexperimentalfeatures" Set that to true, then find the world you were playing on, then click "Seed" on that world. It should give you the seed of the world.
Pericles_Corde
2049022970 - That's the seed. Hopefully mods don't matter, then again I only used QoL mods and the throwing expansion. No world altering ones.
Stardust Guardian
thank you! I will see if the seed works with unmodded. Well it should, but if I see the musharnite caves
Pericles_Corde
http://i.epvpimg.com/nGS5cab.jpg It's around in this area. Botton right one.
Stardust Guardian
Thank you for the location! You got your hands on a really rare type of world gen. One time, I Got a spider and mushroom biome mixed
Pericles_Corde
But I bet there were no glowing spiders xD