Diamond City Diamond City , commonly referred to as " The Great Green Jewel , " is a fortified settlement in the Commonwealth . This settlement is based on the pre - War structure of , its distinctive " Green Monster " color visible on the reinforced walls . The city guards can be seen wearing baseball jerseys with umpire ' s pads and helmets , reflecting the location ' s former use . Background . Diamond City was founded in the 2130 ' s in Fenway Park and quickly established itself as a safe haven for settlers of the Commonwealth , thanks to what its residents affectionately referred to as " the Wall . " Later decades saw Diamond City grow into the Commonwealth ' s largest and most prosperous community . It developed a degree of social stratification as wealthier residents came to occupy properties in the settlement ' s exclusive upper stands , these overlooked the lower field where everyone else strived to make a living . In 2180 , the Commonwealth Minutemen defended the city from an attacking horde of super mutants . This caused the faction ' s eventual rise to fame . The " Broken Mask incident " occurred in 2229 , when a man calling himself Mr . Carter arrived . At first , the people of the city warmed to Mr . Carter , but while he was drinking at the bar that occupied the center of Diamond City ' s marketplace , he went berserk and killed several residents . When security personnel killed Mr . Carter , they discovered he was a synth created by the Institute . From that moment on , the people of Diamond City lived in fear that they would be covertly replaced by synths . Although such talk was discouraged and ridiculed by the city ' s leadership a number of people began inexplicably disappearing from the city over the following years , a disturbing trend that no one wished to acknowledge . In 2272 , the protectron Takahashi took up residence in the marketplace ' s old bar and opened Power Noodles , a restaurant that became a staple of Diamond City culture . By 2282 , Diamond City ' s mayor was McDonough , whose controversial election campaign centered on the expulsion of all ghoul s from the city . Upon election to office , the decree was enacted and the ghouls were driven out by the human residents . Some ghouls relocated to Goodneighbor and continued to bear a grudge against both Mayor McDonough and the inhabitants of Diamond City in general , who did little to reverse the decree . By 2287 , some residents came to believe McDonough had been replaced by a synth , a suspicion exacerbated by Piper , her sister Nat and their newspaper " Publick Occurrences " . When Piper reaches idolized status with the Sole Survivor , she explains that when she first moved to Diamond City with Nat , the Great Green Wall was broken and patched with a solitary bookcase . This drove her to create " Publick Occurrences " to address the issues of Diamond City . The city is split into social classes where the rich and powerful reside in the upper stands , whilst the common people live in the field below . At the back and to the side of the stadium there is a crop field and brahmin pen , similar to the one in Abernathy Farm . There is also a small lake to the right of the marketplace , which is irradiated . Appearances . Diamond City appears only in " Fallout 4 " and is also mentioned in Zoe ' s diary in the " Automatron " add - on . It is also a mentioned location as a question in the " Fallout Shelter " weekly quest Game Show Gauntlet .