Acid is the second most advanced form of liquid. Players should be cautious when dealing with acid, because it kills the player much quicker than plain water. There are different forms of acid in FE2 maps, like the purple-colored acid in Dark Sci-Forest , the magenta-colored acid in Relic Valley and the poison in Poisonous Valley.
Acid is mostly green in color, giving a more realistic appearance of imagined acid. Like it may be in real life, acid decreases your air much faster than plain water, but still doesn't kill you immediately. Many maps require you to swim through acid to get to a destination. This is a tactic used by many mapmakers that provides an extra challenge to the map.
Sometimes, at a certain point the water will turn into acid and start decreasing the player's air very quickly. Make sure to quickly reach your next area because you will lose ~30 air per second in acid. If you're swimming in acid, then the air meter will show a ⚠️ symbol and your air is colored red while decreasing signifying the fast air loss. (Previously it showed "⚠ ACID ⚠"). Acid is most common on Easy, Normal, and Hard maps. Later on in most maps, the acid will turn into lava. You can learn more about lava here, or you can learn more about water here.
A unique feature about acid is that while you swim in it, your air will begin converting to "poison air" which is represented by a red portion of the air meter (high detail) or a red number on the side (low detail, hidden menu). While submerged in acid, air converts to "poison air" until you have 20 "poisoned air" after a second. If you reenter the acid after gaining "poison air", all "poison air" will be deducted instantaneously, possibly resulting in instant death. When regaining air after exiting acid for 1 second, the "poison air" will convert back to regular air at ~5 air per second. Be careful when dipping in and out of acid because poison air can cause very fast air loss, especially on acid-intensive maps like Dark Sci-Forest.
- Acid can kill in about 3.33 seconds normally, and in about 13.3 seconds with an Air Bubble.
- In an old version of Cave System, the acid turns itself back to water in order to access the swimming area, possibly because button functions via EventScript never existed at that time. This occurrence is very rare on.
- The poison air mechanic was added in an early FE2 update. Before, acid just drained air faster.
- Acid used to be called "Toxic Water" as shown in an old screenshot  and video of Abandoned Facility.