2018-01-19 : Enter the Unknown’s effect allows you to play an additional land during your main phase. Doing so follows the normal timing rules for playing lands.

2018-01-19 : If no card is revealed, most likely because that player’s library is empty, the exploring creature receives a +1/+1 counter.

2018-01-19 : If the target creature is an illegal target by the time Enter the Unknown tries to resolve, the spell doesn’t resolve. It won’t explore, and you won’t be able to play an additional land.

2018-01-19 : If you somehow manage to cast Enter the Unknown when it’s not your turn, the target creature explores when it resolves, but you won’t be able to play a land that turn.

2018-01-19 : Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until it’s done. Notably, opponents can’t try to remove the exploring creature after you reveal a nonland card but before it receives a counter.

2018-01-19 : The effects of multiples of Enter the Unknown in the same turn are cumulative. They’re also cumulative with other effects that let you play additional lands, such as the one from Wayward Swordtooth.