Text (US+errata): You may play an additional land on each of your turns. [Oracle 2009/05/30]
You have to say when you put the land out that it is the one you get because of Exploration. [bethmo 1998/10/23] If you put out a Forest while Exploration is in play, then Exploration leaves play (by being destroyed or something), you cannot put out another Forest claiming that the first one was due to Exploration unless you said so when you played that Forest.
