PRAGUE  One Sunday, several months ago, early risers gazing at Czech Television’s CT2 channel saw picturesque panoramas of the Czech countryside, broadcast to the wordless accompaniment of elevator music. It was the usual narcoleptic morning weather show.

Then came the nuclear blast.

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.

Now half a dozen members of the group face up to three years in jail or a fine or both, charged with scaremongering and attempted scaremongering. The trial is set for March. Some Czechs expressed outrage over Ztohoven’s action, naturally, but in general it drew a mild, tolerant, even amused public response, in contrast to how terrorism-related pranks, or what might seem like them, have been widely greeted elsewhere. The incident instead has highlighted an old Czech tradition of tomfoolery that is a particular matter of national cultural pride.