Text (4th+errata): Destroy X target Mountains. ~this~ deals damage to each creature and each player equal to the number of Mountains destroyed this way. [Oracle 2008/01/01]
Can be used with X equal to zero. This is useful if no Mountains are in play. [bethmo 1994/06/01]
Casting Magical Hack on Volcanic Eruption will not allow you to change the targets of the spell because you chose the targets when the spell was declared and before a Hack can interrupt it. The Hack will just cause it to be countered since the targets will be illegal. [WotC Rules Team 1994/02/07] It may not be countered on multilands that are half mountain and half of the new land type specified.
Extended tournaments (see Rule 913) have banned this card since 1999/10/01.