You can unlock this once you have a level 5 radiation suit, are in California, and have located Hollywood. So put on some Kool & the Gang and get busy.
This achievement is straightforward once Redondo Beach is available. However, there are no clues on how to do this, so that's the tricky part.
So what you need to do is gain access to the Hollywood Sewers, which you can get to by going to Hollywood, then by going through the Hotel California. You'll have to kill some junkies, but this has no effect on the game. Once you're in the sewers head for the southwest corner. Make sure you save before you get there, just in case you mess up. Eventually you'll come to a junction with two boxes on the left, two Salt Thug's straight ahead, and a lengthy passage to the right.
Want you should do now is loot the two boxes (they need to be unlocked first), then send a lone character to eavesdrop on the aforementioned Salt Thugs. Fairly soon they'll mention checking on the two boxes you just looted. Retreat down the long passage (not the way you came) and chill out a safe distance so you can continue to eavesdrop on these two crusty dudes. They'll speculate that another crusty dude with the unlikely name of Bob probably took their gear. This will prompt a log book update that will effectively open access to Redondo Beach. As far as I know, this is the only way to gain access to this location.
*** SIDE NOTE *** You can also unlock the "Pushed the Button" achievement in this same location. The Salt Thugs will go back to their previous spot guarding a breakable wall. Surprise and kill them like the ruthless Ranger you are, break through the wall (you can use Brute Force, but shooting it is quicker). Proceed down this corridor to the glowing red button at the end. Make like Salt n Pepa and push it. Achievement unlocked. If you want to know what it does, read this:http://kotaku.com/it-took-a-year-to-find-out-what-the-red-bu...
Suffice to say it doesn't do anything you need to worry about. *** END SIDE NOTE ***
Now, simply exit the sewers and Hollywood and head to Redondo Beach n the southwest corner of the map. Once there, traipse around and do your best Dig Dug impression. There are a lot of holes, so pretend you're Shia Labeouf pre-super freaking weird phase(s) and excavate! It'll take a while. The achievement unlocked for me with one hole left to go. What a timesaver.