Richard Marks, 50, the superintendent of Grand Canyon National Park, stepped aboard a 19-ft. Boston Whaler last month to hear it for himself. Skimming eastward across the azure expanses of Lake Mead, Marks and two park rangers maneuvered their craft into the lower reaches of the Colorado River. Soon the majestic red walls of the Grand Canyon towered overhead. They cut the engine, grounded the boat on a sandbar and waited quietly.
"It wasn't very long before I saw it," said Marks. As a concussive whumping sound crescendoed around them, the helicopter skimmed 100 ft. over the river, then dodged out...
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