Geocaching

Geocaching is a quest given by the Mysterious Radio Signal. The player must be level 15 before starting this quest.

Walkthrough
There are 4 transmitters spread across the Valley Wasteland that broadcast the Mysterious Radio Signal. The Pip-Boy radio can be used to locate these transmitters. Each one contains a note with 4 digits of a 16 digit code and 1 digit of a set of coordinates. Find all of the transmitters and head to the coordinates and you'll discover an old pre-war bunker. Enter the code at the terminal to go inside.

The inside of the bunker is essentially a death maze, filled with traps, radioactive waste and feral ghouls. At the end of the maze is a ghoul wearing power armor that asks for the player's name and introduces himself as Sergeant William Lancaster. When the bombs dropped, his company hid inside the bunker. It protected them from the explosion but not fully from the radiation, and they all became ghouls. They all went feral except for Lancaster, who decided to take his feral ghoul pack roaming the Wasteland and gathering loot. Eventually he bored of this and created a system to trick loot-laden adventurers right into his death trap. He even keeps a list of the names of everyone he's killed.

The simplest solution to the quest is to kill Lancaster. Alternatively, you can convince him to leave his treasure behind and get to know people in the Wasteland (Speech 75).

Rewards
Lancaster's treasure hoard

T-45 Power Armor, if you kill Lancaster