Choose any number of target players. For each of those players, reveal cards from the top of your library until you reveal a nonland card. Kaboom deals damage equal to that card's converted mana cost to that player, then you put the revealed cards on the bottom of your library in any order.
Official Rulings for Kaboom!
10/4/2004 : If all the cards in your library are lands, this card deals zero damage.
10/4/2004 : If you target more than one player, you do the whole thing for each player and you do each player in turn order.
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