Green Mushroom

Most underground plants pop off the moment anything touches them. This one does not. Swinging a sword through it accomplishes nothing, and you have to actually mine it with a pickaxe, which is why plenty of players walk past these for a whole playthrough without registering that they are collectible.

The Spelunker buff highlights it along with the other dye plants, and that is by far the easiest way to gather a supply. Run a potion while you are tunneling for ore and pick up the mushrooms on the way past.

It takes root on ordinary dirt, stone, mud and jungle grass down in the caverns, but it ignores the converted stone types, so Ebonstone, Crimstone and Pearlstone are all dead ground for it. In a world that has been chewed through by Hardmode spread, the area that can still produce it shrinks steadily. Its only real purpose is Green Dye.