Exeter: Imagine an entire civilization existing within the United States... in Secret

Imagine an entire civilization existing within the United States...in secret. Technologically advanced beyond our imagination, this society has created a utopian community in hidden mountain caverns.
Exeter...a community of warriors wanting to live in peace...choosing centuries ago to hide themselves from the outside world.
Through world-wide network of communications, Exeter discovers an impending terrorist attack on the United States.
Now the dilemma...does Exeter notify the government and risk revealing themselves or allow the destruction of a major U.S. city?
They come up with another option...stop the attack themselves...in secret.