One of the Ambient Structures from Katters Structures.
Underground Cave with Mushrooms growing in it
Quite bright for its position underground
The main blocks are :
No Loot
Shroom Cave, will naturally spawn in almost every Biome, except Biomes that have water surfaces and Underground Biomes like the Deep Dark, and corresponding Biomes O' Plenty and Terralith Biomes.
Mooshrooms will naturally spawn here
No Hostile Monster can spawn in it