Being able to know what lies submerged beneath the surface of the water probably would have saved legions of sailors and their craft in the thousands of years man has been going to sea in ships. Submerged rocks, jagged reefs, treacherous sand bars—all have taken their toll of men and boats because skippers were not aware of such hazards to navigation until it was too late. Shoal waters are the graveyards of vessels that came to grief because they were off course with no instruments so those aboard could discover the error.
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