Oracle Text

Choose any number of target players or planeswalkers. For each of them, 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 or planeswalker, then you put the revealed cards on the bottom of your library in any order.

Card Rulings

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 process for each player in turn order.