In Gone, the FAYZ occurs, making all the adults disappear. At this point, it is unknown whether they have been transported outside of the FAYZ, somewhere in the FAYZ or perhaps, as Astrid suggests, ceased to exist or were perhaps transported into another universe.
In Lies, Orsay goes into the dream of Constance Temple (Sam's mother). She learns that the twins have appeared on the outside and no other kid since. It is also shown that Connie is one of two spokespeople for the adults awaiting their kids' to return from the FAYZ (Or the bowl as they call it). At the end, the wall disappears and the outside is shown for a few moments.
In Fear, the outside becomes a part of the book as nearly every few chapters is shows life outside and the troubles of Connie Temple. It mentions that the dome was being monitored by scientific teams. Later, Connie Temple discovers that the teams were planning to detonate a nuclear bomb beneath the FAYZ, in hopes of damaging the barrier. However, the bomb does not damage the FAYZ. The FAYZ itself then turned transparent, revealing the people inside. The FAYZ's transparency was probably caused by this detonation, although this is not confirmed, although it would explain why the nuclear bomb did not cause any damage to the outside world - this energy was absorbed by the barrier.
In Light, all of the parents are now camping at the barrier on the Highway in order to be near their children. However most of the outside world no longer wants the children to be freed. People are convinced the kids are all dangerous criminals. Many people come to the site of the anomaly with many cameras which capture the dreadful conditions inside the FAYZ. People outside have seen footage of children carrying weapons and Sam burning Penny's body and setting Gaia on fire. Due to that people even suggest that "next time they should use a more powerful bomb". The people outside have even set up a big TV screen with captions for the children to watch. Google has even relocated one of their satellites in order to watch the barrier and the Daily Show has interviewed people about the conditions inside the dome.
Later on, once Little Pete and Gaia both are killed the barrier vanishes, releasing the children into the outside world.