Radioactive Decay

 is a new gameplay mechanic that some Bloons have. Radioactive Decay causes Bloons to take some damage, typically 1% of their remaining HP, every few seconds. After taking a certain amount of damage from decay, typically 15%, the Bloon that decayed will take massive damage and release other types of Bloons that are typically not their children.

You can find all decayable Bloons here.