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 Fenway Park, 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 ghouls 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By 2287, some residents came to believe McDonough had been replaced by a synth, a suspicion exacerbated by Piper, her sister Nat and their newspaper Publick Occurrences.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.

Behind the scenes
Atop the press box, there are five red flags reading "World Series Champions" with the years 1903, 1912, 1915, 1916, and 1918. This implies that following the Divergence, the Red Sox never won the World Series again and the Curse of the Bambino was never broken. In the real world, the Red Sox finally won the World Series after an 86-year drought in 2004.
 * According to the left field bleachers, the team's retired numbers in the Fallout universe were 1, 4, 6, 11, 13, 21 and 33. Of these, 1, 4, and 6 are actual retired numbers.
 * Diamond City resident Moe Cronin bears a name near identical to former Red Sox player Joe Cronin, owner of one of the aforementioned retired numbers. Ironically, Moe's idea of what the sport of baseball is and how it's played is comically skewed.
 * The scoreboard which Diamond City residents revere as "The Wall" is a real feature of Fenway Park's design. Its real nickname is "The Green Monster."
 * In the real life Fenway Park, there is a large Coca-Cola sign approximately in the same location and style as the Nuka-Cola sign in game.
 * Likewise, the Mass Fusion billboard mounted on a building behind The Wall mirrors the placement of a large advertisement for the Citgo gasoline company made of neon lights. The real sign was rigged so that it could light up the letters C, IT and GO in sequence ("See it go") when a home run was hit.
 * There is a single red seat in the bleachers, representing where Ted Williams hit the furthest ball in Fenway Park history, 502 feet from home plate.

Bugs

 * When using the elevator to the mayor's office with a companion the companion can end up at the first green trash can when you enter Diamond City, behind the street light in front of the elevator and be stuck there. Walking outside of Diamond City to the Fens (manually without fast-travel) will sometimes work to free a companion in this frustrating situation.
 * For PC players, utilizing after selecting the companions ID can rectify this issue.
 * Occasionally, if the player activates the elevator on the way down from the mayor's office, and before the elevator reaches the bottom floor, activates the second button, the guard rail will extend, as if the player were to disembark, and retract upon arrival, as if the player is riding the elevator.
 * Activating the elevator and leaving Diamond City or entering a building before the elevator stops moving and reload the game may fix this. On PC Using to noclip through the exit will make this easier.
 * Putting any items in mailboxes, even the one in front of the Sole Survivor's Home Plate residence, will show as owned upon closing the dialogue box. Stealing is the only way to access said items.
 * After completing the quest Unlikely Valentine, two triggermen may spawn in Diamond City each time you travel to the city. Nobody in Diamond City will engage, leaving the Sole Survivor to eliminate the two themselves. As of now, there is no fix.