While Magellan is not hidden, it is easily fixed. To get this achievement, do the following:
Go into your Steam folder. From there, follow this path:
SteamApps -> common -> sid meier's civilization 5 -> Assets -> Gameplay -> XML
Open up the GlobalDefines.xml with Notepad.
Find the line:
CIRCUMNAVIGATE_FREE_MOVES
You will find "<value>0</value>" there. Set it to 1.
Then, be the first to circumnavigate a map. As soon as you did this, the Magellan achievement will be unlocked.
I recommend you switching the value back to 0 once you unlocked it, as it is altering the gameplay. Originally, circumventing the world as first would give all of your ships a +1 movement bonus. I suppose Firaxis thought that would be problematic, so they removed it. Since the achievement is tied to this bonus, you can see why it won't work.