Find the missing people for Marcus

In Broken Hills, go to the east side of town. There are a few manholes in this area (one of them in an outhouse). Go down, kill a few ants, and locate the bodies in the NE corner of the tunnels. On one of the bodies, there's a note from 'Francis' that implicates him as the killer. Once Go see Marcus, the mutant town sheriff, and ask him to do this quest.

Afterwards, locate a man named Don in a house in the north-east corner of Broken Hills. He will tell you that his wife is missing, and you will be able to find her and get some XP.

Being in the east side of town, find a manhole (there's one in an outhouse), there are a few in that area.

You will have to face a few giant ants, but they're probably nothing you can't take on by now. Then in the tunnel, locate the mass amount of dead bodies, and find the body of a woman (more precisely, Don's wife). There will be a note on her body.

The note implies that Francis is the killer of these missing people.

Go back to Don, and tell him you found his missing wife... er, dead. You will receive 500 XP for this. Then talk to Marcus to get more.

Also, if you have defeated Francis in arm-wrestling, you can confront him with the note you found (yay, more XP) and then he will leave the town.