In Fallout 3, you can raze an entire city to the ground during a notorious quest. But Megaton is apparently not completely lost afterwards, even though that was not intended by Bethesda.
If you want to, you can be incredibly evil in Fallout: In Fallout 4, it’s possible to sell a child into slavery or take over entire settlements with raider gangs. In Fallout: New Vegas, you can turn a solar power plant into a superweapon, and in Fallout 3, you can even destroy an entire city with a nuclear bomb if you feel like it.
The settlement of Megaton is reduced to little more than a crater. But you can still visit it in its undestroyed form via a secret entrance.
Returning to the ghost town
A Fallout player shared his discovery on Reddit and explained what happened: After he had destroyed Megaton, he noticed that a quest he had received from a resident of the city was still active. In fact, the game is designed so that the quest giver, named Moira, survives the explosion.
TIL: you can enter Megaton even after blowing it out
byu/GANJAM3N420 inFallout
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The story of Megaton
Megaton is a fortified city in Fallout 3, built around an undestroyed atomic bomb. This pre-war relic is worshipped by the Children of Atom in the city and viewed with suspicion by everyone else. You can defuse the bomb for the sheriff of Megaton or side with the unscrupulous Allistair Tenpenny and detonate the city with the help of the bomb.
However, she usually lives in the settlement of Underworld. In this case, however, the quest marker led right into the wasteland and pointed several meters into the ground.
Using a console command (on the PC, you can use a console to use cheats and thus walk through walls or the ground, for example), the player dug through to the marker and discovered a manhole cover.
When he opened it, he landed in a loading screen and suddenly stood in the undestroyed Megaton, which had actually been disassembled into its atomic components. Eerily, however, all the inhabitants of the ghost town had disappeared except for Moira and a Brahmin.
The existence of Ghost Megaton is of course no hidden lore secret, but simply an oversight: it is not without reason that access to the undestroyed city cannot be reached without console commands. And it can only be found because Moira did not end up in her new home Underworld after the explosion. She was probably too attached to her city, which was so callously destroyed.