Flammable Gas is a non-toxic, green gaseous Material emitted from Acid corrosion and from players with the Gassy Status Effect. It is extremely flammable and gathers in large pockets as it floats up into the ceiling.
As Acid is fairly prevalent throughout the game this gas can be found in most biomes. Sealed rooms filled with concentrated amounts of it can be found in the Fungal Caverns, usually near several pedestals with Flasks, and certain structures in the Snowy Depths. When flammable gas is generated as part of a static structure like this, it will be generated as a static, motionless variant similar to normal Flammable Gas.