"The world's first cake avalanche," warned Belmont, America's Master of Magic, and some in the crowd went "Ohhhhhhh."

"The leaning tower of Tysons Corner," Belmont continued, and a wan smile flitted across the face of Karl Seibt, the master baker who with three assistants had spent nearly 200 working hours creating the 12-foot-tall, 1,200-plus-pound, 122-tier confection.

"It is what you call in the baking business 'a dangerous tilt,'" Belmont told the lengthening lines of celebrantsyesterday at Tysons Corner Center, where 144 merchants were letting 12th anniversary of the shopping center's opening.

Showing onimous signs at one point of caving in under its own weight ("The last layer is a little too fresh," confided Seibt), the cake was propped up by Seibt and others until Bruce Rosenberger could cut the first slice.

Rosenberger, who is 12 (was there any other number yesterday?), was accorded cake-cutting honors because a dozen years earlier his mother, then pregnant with him, had stood in a similar line at the opening and as a result, Jackie Rosenberger gave birth to Bruce 10 days before he was due. Apparently merchants decided that such devotion was not to go unrewarded forever.

Seibt, who was executive chef at Chez Francois in Washington for four years before he opened Seibt's Konditorei first in Vienna, Va., and later at Tysons , started making 1,200 spun-sugar roses for the cake 12 days ahead. Five hundred cups of flour, 272 cups ofshortening, 140 cups of butter, 12 cups of baking powder, 12 cups of salt, 290 cups of milk, 1,540 eggs, 1,058 cups of sugar and 16 cups of vanilla later, the 5-million-calorie piece de resistance stood sentinel in Tysons' Aviary Court, awaiting the multitudes.

Their first bites were cautious ones, since hidden among the layers were 12 dozen automobile ignition keys, one of which fits a new Ford Pinto being given away for the occasion. Today, inthe grand old tradition of Cinderella 144 cake eaters are expected to show up at Tysons to see which of them has the key to success -- and who simply has a sweet tooth.