There's some scientific disagreement today about how big the New Madrid quakes of 1811 and 1812 really were. Did they hit the monstrous 8-plus level, which would have made them stronger than the 1906 San Francisco quake, or were they closer to 7, as a team of university researchers calculated a few years ago? Either way, it didn't matter much in the early 19th Century, when the area was mostly wilderness. Today, in the worst-case scenario, an 8-pointer would wreak havoc over a much more densely populated 5-state area. Particularly at risk is Memphis, Tenn. 35 miles west of the epicenter of a 6.8 quake in 1843. Researchers have warned that a huge quake might cause widespread flooding in the low-lying city and destroy a major bridge, in addition to damaging scores of buildings and endangering oil and natural gas pipelines. A 1990 federal disaster drill projected as many as 2,200 dead and 18,000 injured. Property damage? Perhaps as high as $40 billion.