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ospherules, carbon spherules, nanodiamonds, fullerenes, charcoal, and soot. The layer's composition may indicate that a massive body, possibly a comet, exploded in the atmosphere over the Laurentide Ice Sheet 12,900 years ago. The timing coincides with a great die-off of mammoths and other North American megafauna and the onset of a period of cooling in Northern Europe and elswhere known as the Younger Dryas Event. The American Clovis culture appears to have been dramatically effected, even terminated, at this same time. Speakers will discuss numerous lines of evidence contributing to the impact hypothesis. The nature and frequency of this new kind of impact event could have major implications for our understanding of extinctions and climate change.