From Halopedia, the Halo wiki
Pirth was the metropolitan capital of Arcadia. The city was surrounded by a swamp, which housed part of the Arcadia ONI facility and several Forerunner structures including secondary reactors. The city also has a large dock area, which was densely built-up.
Pirth City was one of several cities attacked by the Covenant during the Battle for Arcadia in 2531. During the battle, the city's location was quickly discerned by the Covenant, who in turn focused a significant amount of their local resources in an effort to raze the city.
Pirth City was abandoned when Arcadia's inhabitants scattered, either fleeing the planet for other colonies or retreating to the last major bastion of civilization on the planet: the lawless community of Abaskun. It was abandoned, and eventually became overgrown as a jungle encroached upon the once bustling metropolis. It was most likely glassed with the rest of the planet when the Covenant returned.
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