Great Barrier

The Great Barrier is shaped like a giant dome, where inside a giant mist has formed.

"Shield" and Relevance.
The anime show explains at the beginning how the president has taken "emergency measures," which were not elaborated on until episode 2.

Functionality
Further elaboration in episode 9 shows that a C-17 aircraft was able to fly through the barrier unharmed due to unnatural works surrounding the 'Fog.' However, the giant tentacle effect is not explained thus far in the series.