From the NYtimes:
Across the Krkonose Mountains, or so it appeared, a white flash was followed by the spectacle of a rising mushroom cloud. A Web address at the bottom of the screen said Ztohoven.com.
Ztohoven, to no one’s great surprise, turned out to be a collective of young artists and friends who had previously tinkered with a giant neon sculpture of a heart high atop Prague Castle, and managed (during a single night, no less) to insert announcements for an art opening inside all 750 lighted advertising boxes in the city’s subway system.
To hack into the CT2 broadcast, Ztohoven simply switched cables on an unmanned, remote camera at a limestone quarry in the mountains, which the artists had scouted three years earlier. Then they piped in their video. The name Ztohoven makes a pun in Czech that means both “out of it” and an obscenity. Rightly, the group presumed this would tip off viewers that the explosion was fake, in case they hadn’t already guessed it from the cheesy special effects.
Now they face prison sentences for "attempted scaremongering".