personaly,having found very old shoes, harness leather, beaded moccasins, and other probable organic artifacts, beach bone anyone? in a variety of useualy anoxic or acidic situations, I can then extrapolate, that these are common things to find if a person takes a moment to examine and confirm that they are historical artifacts, and all in all the top 10 to 100 feet of most of our planet is a good place to look for direct evidence of past human activity, use a microscope and now 100% of the planets surface has an artifact.