About 20 kilometers southwest of New Orleans, one of the U.S. Geological Survey's benchmarks sits atop a concrete column that pokes above the waves about 5 meters from the shore of Couba Island. The small brass disk, one of thousands that the agency has installed throughout the country, serves as a reference point for surveyors and mapmakers. Why did agency personnel place what should be a readily accessible guidepost in the thigh-deep waters of a Louisiana bayou? The answer's easy: They didn't. When the disk was installed in 1932, it sat high and dry in someone's backyard.