Sentinel of the Nameless City

If no card is revealed, most likely because that player's library is empty, the exploring creature receives a +1/+1 counter.

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.

Map tokens are a kind of predefined token. Each one is a colorless artifact with the artifact subtype Map and the ability " 1 generic mana , tap , Sacrifice this artifact: Target creature you control explores. Activate only as a sorcery."