Metropolitan Los Angeles is sitting on ground zero. Slicing under the highrises of downtown is a network of potentially deadly faults that could trigger the nations's most devastating natural disaster. On January 17, 1994, a previously unknown fault convulsed beneath Northridge. It wa sthe most destructive earthquake in modern Los Angeles history. NOVA takes viewers from urban trench sites to the top of the Santa Susanna Mountains, in search of clues to where nature will strike next.