Mushrooms is a key ingredient in upgrading Scrappy to its final tier. While they are particularly hard to find, there are two maps where you are guaranteed to find them - Dam Battlegrounds and Spaceport. Here's what you need to know to farm mushrooms in Arc Raiders.
Near Little Hanger in Spaceport (Guaranteed)
On the Spaceport map, make your way to the Little Hanger (given in map below).

Once you get to the Blue construction site, go below to the swampy region.

Look for the half stump of a tree.

Go to the tree and you'll find several mushrooms to harvest. Mushrooms spawn on this tree just about every time. So you can never go wrong when farming for mushrooms here.

Hydroponic dome in Dam Battlegrounds
Head to the Dam Battlegrounds map, near the Hydroponic Dome north of water-treatment. This area provides a lot of mushrooms.

Mushrooms tend to grow on the water line. So make sure to walk on the edges of the boggy pools to find them. Look for fallen trees and stumps. If not inside, check outside. Skirt the water lines and find fallen trees there.

The Wreckage area near the Scrap Yard and the Testing Annex is also known to spawn mushrooms. Look for fallen trees and debris near the carcass of a giant Baron ARC.
These two areas on the two maps - Spaceport and Dam Battlegrounds - provide guaranteed mushroom spawns. Whether you're saving them for Scrappy upgrades or consuming them for health, you can never go wrong with your mushroom runs if you make them on these two maps.
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