The so-called Blue Area of the Moon is a region on Earth's moon which through advanced alien technology maintains a breathable atmosphere with enough atmospheric pressure to enable people from Earth (and similar worlds) to survive without needing pressure suits or carrying air with them. The Blue Area sits in the Luther crater in the Mare Serenitatis and is visible from Earth through binoculars or a telescope, though it does not have a blue tint to it when viewed from Earth; the name "Blue Area" comes from the fact that the sky in this area of the moon appears blue during the two week long "day" (due to having a similar chemical makeup as Earth's atmosphere).
Over twenty thousand years ago, the alien Kree discovered Earth and the fledgling human species. Presumably it was the Kree who created the Blue Area, as they built a watch-post there which grew into a small city. At this time, Kree scientists took a tribe of humans and performed experiments on them, turning them into the Inhuman sub-species. At some unspecified point after this, the Kree abandoned the Blue Area, though they left a sentry robot in standby there to alert them if and when humans would discover the ruins.
The Blue Area was first charted by humans during the Apollo 9 and 10 missions, which orbited the moon without landing. It wasn't until the Apollo 13 mission when people first landed, only to be attacked by the Kree sentry (the failure of the Apollo 13 mission has since been the subject of a NASA cover up, which was reported as being a problem while en route). The Apollo 18 mission was scheduled to land just outside the Blue Area and explore it on foot, but the mission was scrapped by NASA officials reviewing the Apollo 13 footage of the area. Instead, both NASA and the Soviet Space Agency sent unmanned probes to study the area, collecting data on the air there.
It wasn't until the 1980s that the existence of the Blue Area was admitted to exist by space agencies across the world. Despite a push to explore (and colonize) the Blue Area, it remained a scientific curiosity rather than a subject of serious study. (Many have accused the government of being too afraid to acknowledge the implications if it was indeed artificial.) It was only the recent migration of the Inhuman city of Attilan to the southeast corner of the Blue Area, recorded by astronomers across the world, that spurred serious scientific interest in the Blue Area.
The first known people to visit the Blue Area besides the Inhumans were the Fantastic Four, who flew their starship there on behalf of NASA. While the Four noted the newer city in the distance, they were more interested in the ruins of the older city that dominated the crater. The Four accidentally activated the Kree sentry robot, which tried to capture them; it was not prepared for their powers, however, and was swiftly defeated. The Kree's response to the sentry's defeat is unknown, but has likely spurred Colonel Yon-Rogg to accelerate his own timetable to conquer the planet for the Kree.
Other than the Inhumans, the entity known as Uatu the Watcher makes his abode in the Blue Area. It is not known whether humans will attempt to colonize the Blue Area.
First Post-Reboot Appearance: FANTASTIC FOUR #