Nature of the sponge-worlds is very complicated, it reacts to presence of a big amount of mentally active subjects and causes the Reaction of Psychosphere. In other words the substance itself that is the basis of all worlds reshapes into different monsters and attacks foreigners. These attacks take place at regular intervals and depend on the player's behavior. The religious society of the Frames called this phenomenon Filth. According to the religious theory the whole Worlds' Chain is a Purgatory for people, a way for mankind to purify.
On some worlds Filth is very active so the missions are entirely dedicated to finding the way to survive in these hostile conditions. During other missions Filth plays an important part of a "third party". In most of the cases the force field (Perimeter) becomes the sole form of protection from Filth attacks. Some Frames have learned to tame natural power of Filth and use them to suit their own ends.
There are about ten types of Filth. These are ground-based, air, subterranean and other units. All of them behave differently and have different aims but they always activate at regular intervals. Players should always take Filth into account and activate the shield to protect themselves.