After the prison break, talk to Jacob, and he will 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 are automatically 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.
Your karma will drop fifty-points, killing off every super-mutant in the town, and dooming the survivors.
Any quests involving Marcus or Francis can't be completed after this since they were killed.
Once you receive this quest, you can instead turn the gang in to Marcus for 2500 xp.