Imagine there's no Imagine Mosaic (it's easy if you try): The centerpiece of Central Park's Strawberry Fields is in danger of collapsing. The first cracks appeared in the 25-year-old memorial five or six years ago, according to M.C. Reiley, the Central Park Conservancy's supervisor of monuments conservation, and what he called the "last, best chance" to save it came last week. "The problem is that it wasn't constructed very well," said Reiley, who is also a sculptor. The mosaic is eleven feet in diameter but sits on a concrete slab only ten feet across. "So, right off the bat there's been this problem with a half-foot of the mosaic all the way around not resting on anything," Reiley explained.