One-Man Scouting Party

 is an Fallout 3 achievement on Xbox 360 and PC and a trophy on PlayStation 3.

It is granted for discovering 100 locations. You must literally go to the 100 locations, the explorer perk puts all locations on your map but it does not discover them for you, once you travel to the location, the symbol on your pip boy will invert color and it will be considered discovered.

Strategy
If you have a save around late level 19, one method is to level up and utilize the explorer perk, allowing you to have all locations revealed, once this is done, load up with weapons and ammo and head into the wastes going from location to nearest location until you get your achievement, condensed city centers are the best place for fast discovery as they are packed together and have many per instance.