From Halopedia, the Halo wiki
Topography and climate
Cassidy III is a typical large, blue-green world with swirling clouds white and clean as fresh snow. From the surface, the sky appears a shade of indigo during the day. Both the planet and is atmosphere look pristine from orbit, drawing comparisons to pre-industrial Earth. Only one colony ever settled the planet and its colonists have not done much to spoil it. It has one large continent surrounded by concentric rings of islands that form long and fertile archipelagos. The mainland is bisected by a gigantic mountain range with a vast jungle on one side and a sprawling desert on the other. The desert's ridgelines and sand can reflect sunlight to give them a glowing red appearance. The Hole in the Wall is located on the lush and golden high plains just on the desert side of the mountains, sitting on land that is fairly flat apart from a dry and jagged ridgeline to the south. High and dry but not desiccated, the colony is gathered around a trio of massive Forerunner towers on the edge of a large lake. There the air smells like freshly turned soil with just a hint of a breeze off the lake. On the opposite side of the planet from the Hole, not a single artificial light can be seen when it is in darkness, though a line of active volcanoes there glows red-hot.
Prior to the firing of the Halo Array in 97,445 BCE, the planet later known as Cassidy III, Cybele VI, or Freedom was frequented by Forerunners, who built various structures on its surface which were embedded into the landscape.
Rise of humanity
When Unified Earth Government pioneer groups initially scouted the planet, it was considered to be highly remote from the standpoint of human-occupied space at the time. So although a UEG colony was established there, it was abandoned by the governing authorities by around 2458, rather soon after it had been settled. This was due to the travel costs associated with the limited Shaw-Fujikawa translight engines available then. The locals moved into the rubble a decade after its abandonment and found themselves sitting atop of a lot of Forerunner technology. From that point on they quietly eked out a self-sufficient living there, at times selling some of the planet's Forerunner tech on the black market. In 2508 the settlement referred to as the Hole in the Wall was founded by a large number of outlaws—mostly pirates—who wished to build a secure hideout for themselves.
Once this was done, these outlaws fell in with a bunch of insurrectionists on the run also in need of a place to hide. When the Insurrection began to wane due to the Covenant's war against humanity, a large portion of the United Rebel Front was folded into the group occupying Cassidy III. Cassidy III's occupants never showed much interest in deciphering the nature of the Forerunner technology which hid them, being either purely indifferent or worried that messing with it could cause it to stop working.
The Office of Naval Intelligence only discovered the Hole in the Wall thanks to an accidental slipspace failure which caused a survey team to stumble upon it during a routine expedition decades prior to 2558. As the war with the Covenant raged on, ONI hid a number of items on and around Cassidy III, partly because they would be safe there but also in order to monitor the rebels present. These were hidden far enough away from the Hole that its denizens never realized they'd been compromised. The world was never seriously leveraged by the UNSC in order to keep from drawing the Covenant to it. Given its unique ability to remain hidden, the transfer of large amounts of UNSC assets to Cassidy III was one of hundreds of back-up plans to be considered in the event that the aliens ever found Earth. And though its uncanny ability to go undetected by sensors may have been advantageous to research, this was never made a priority while the war was still ongoing. What research that was being done then was happening sporadically, after all, as all resources were being pressed toward stopping the genocide of the human race.
Following the cessation of the war ONI elected to keep the Hole's location classified as Top Secret but leave its existing population alone. And even then, researching this backwater world with anomalous Forerunner technology did not take priority over the many problems the UNSC faced regarding refugee populations, colonies on the brink of habitability collapse, and ongoing conflicts. The last few probes it had sent to the system by 2558 had returned with plenty of evidence of the surface's Forerunner technology and indications that the Hole in the Wall apparently still remained the only settlement.
When the artificial intelligence Cortana led an uprising in late October of 2558 to seize control of the galaxy under the auspices of assuming the Mantle of Responsibility, the Hole in the Wall initially escaped her notice. Adjacent colonies, even the smaller ones, were taken completely offline, so to speak, by the attenuation pulses employed by Guardian Custodes under Cortana's control. The fact that the UNSC's sensors hadn't detected a single Guardian signature at Cybele VI made it abundantly clear to ONI that something about the technology on the planet was concealing it even from her despite the Forerunner resources she had acquired.
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