Swamp Gas Vents can often be found in the Submerged Swamps of the Erebus. They look just like Grass, but with bubbles emerging from the top.
Occasionally, a gas vent will ignite, sending a jet of blue flames upwards that ignites players and mobs but inflicts no inherent damage. They can be compared to the arguably more dangerous Gneiss Gas Vent found in Antlion Overlord Pyramids.
You can acquire them with a tool enchanted with Silk Touch, otherwise they will drop Dirt.