The December Giant

WITH a thunderous roar, hundreds of tons of dirt and rock dropped from sight, tossing trees around like matchsticks and leaving the yawning, lunar-like crater shown above. Now, after investigating the massive cave-in, which occurred last December in central Alabama's Shelby County, the U.S. Geological Survey has identified the crater as a "sinkhole." It may be the largest yet (as much as 425 ft. across and 150 ft. deep) in a growing number of such cave-ins that have pockmarked central and northern Alabama in recent years. Sinkholes often occur when the roofs of underground limestone caverns suddenly collapse. Government scientists are not yet sure what is causing the rash of sinkholes in Alabama (at least 1,000 in Shelby County alone in the past 15 years). But Hydrologist John G. Newton thinks that they may be the result of a natural--or man-induced--lowering of the water table. That would not only remove buoyant support from the subsurface clay above the caverns, but would also cause additional structural damage to the cavern roof by increasing the downward velocity of fresh water seeping into the earth. The eventual result, says Newton, can sometimes be a gargantuan collapse like the one Shelby County's residents call the "December giant."

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