Target creature you control explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on the creature, then put the card back or put it into your graveyard.)
You may play an additional land this turn.
The effects of multiples of Enter the Unknown in the same turn are cumulative. Theyâ€™re also cumulative with other effects that let you play additional lands, such as the one from Wayward Swordtooth.
Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until itâ€™s done. Notably, opponents canâ€™t try to remove the exploring creature after you reveal a nonland card but before it receives a counter.
If you somehow manage to cast Enter the Unknown when itâ€™s not your turn, the target creature explores when it resolves, but you wonâ€™t be able to play a land that turn.
If the target creature is an illegal target by the time Enter the Unknown tries to resolve, the spell doesnâ€™t resolve. It wonâ€™t explore, and you wonâ€™t be able to play an additional land.
If no card is revealed, most likely because that playerâ€™s library is empty, the exploring creature receives a +1/+1 counter.
Enter the Unknownâ€™s effect allows you to play an additional land during your main phase. Doing so follows the normal timing rules for playing lands.