Blow up the mine's air purifier

After doing the prison break, talk to Jacob, and he'll offer this quest to blow up the mine's air purifier and kill the super-mutants. He gives you a detonator, which must be attached to the purifier in the mine. Once this is done, you automatically are transported out of the mine, receive 1500 xp, and every super-mutant in the town is killed. Jacob offers you a small cache of weapons as a reward when you speak to him.

Congratulations! Your karma has taken a fifty-point dive, you killed off every super-mutant in the town, and you doomed the survivors. I hope you feel warm and fuzzy inside.

Any quests involving Marcus or Francis can't be completed after this, since, well, they're dead.

Once you receive this quest, you can instead turn the gang in to Marcus for 2500 xp.