From Halopedia, the Halo wiki
Paxopolis (City of Peace) is the settlement established on the shield world of Onyx by the researchers of the Onyx United Research Project, a joint research project between the Swords of Sanghelios and Unified Earth Government that seeks out valuable Forerunner technology on the world. The settlement houses a spaceport and the headquarters of the research project, headed by Hugo Barton. Some soldiers of the United Nations Space Command took shore leave in the settlement.
Thousands of researchers have willingly agreed to join the project, vetted by the Office of Naval Intelligence, with many intending to spend their life on the shield world. In response to many researchers desiring to bring their families along with them, Paxopolis was established to house the researchers and their families. The settlement houses both human and Sangheili researchers and their families, with Franklin Mendez and Kasha 'Hilot being in charge of Paxopolis' security. Since the settlement's creation, several individuals had already been born in Paxopolis by 2554. By 2558, a number of Unggoy had also taken residence in Paxopolis.
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