Historical Rules · 10 August 2018
Rule 701.39a
Official source text from the 10 August 2018 Rules release
Certain abilities instruct a permanent to explore. To do so, that permanent’s controller reveals the top card of their library. If a land card is revealed this way, that player puts that card into their hand. Otherwise, that player puts a +1/+1 counter on the exploring permanent and may put the revealed card into their graveyard.