Researchers have discovered the remnants of the largest crater of the Great Sahara of North Africa. The double-ringed crater -- which has an outer rim surrounding an inner ring -- is approximately 31 kilometers in diameter. Prior to the latest finding, the Sahara's biggest known crater, in Chad, measured just over 12 kilometers.
Filling mines with the residues of coal combustion is a viable way to dispose of these materials, provided they are placed so as to avoid adverse health and environmental effects, says a new congressionally mandated report from the National Academies' National Research Council.
