Sid Meier's Civilization V Cheats & Codes

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Go into your My Documents folder and look for something like "My Games/Sid Meier's Civilization 5" directory (that should be it, but will depend on where you save your game, so it could be different. Within that directory, locate the file called "config.ini". You can also just search for the file directly and let it find the right location).
Make a backup of that config.ini file (copy and paste), then edit the original with Notepad. Change the line that says:
(Change the zero to a one). Then save the file and restart the game. When playing the game, you can hit the tilde key (~) to show the map.