The answer is: A LOT!
Lately, I have been hunting an old trashy park that has been hunted fairly well through the years. It is one of the largest parks in the city and it has been a park since the 1880’s. The general consensus is that the park is hunted out and too trashy to be worth your time. However, I have been finding wheats and relics lately at the park. Hmmmm…
Then, at the Friendly Metal Detecting Forum (see link on the right), someone pondered what will be left under the ground for future hunters. It is natural for a person to think that in the future, all silver coins and cool old relics will be gone and our future hunters will find only square pulltabs (I hate them!) and clad coins.
Well, I just read an article written by Tom Dankowski (see link to his website on the right) where he makes a startling and eye-opening discovery: Most of the coins under the ground are being masked by iron!
Please read the article yourself and be amazed –http://www.dankowskidetectors.com/behindthemask.htm
Basically, Tom discovered that if an iron target rests above a good target then your detector will see the iron but not the good target. This is called Iron Masking. Furthermore, and more problematic, is what Tom calls Silent Masking. Silent masking occurs when the iron target that is masking the good target is beyond the reach of your detector. That is, the target is either too small or too deep for your machine to detect. In this case, your detector won’t make a beep so you will never know there is a good target under the ground!
Tom estimates that LESS THAN 20 PERCENT of the coins under parks and schoolyards have been extracted because of masking. Wow!
Thank you for looking!
The Dirt Is Good To Me 
Interesting read, thank you!
Bacardi from the forum. Great read, and thanks for sharing. Think I will start changing up my digging habits, and do a little bit of my own expermenting. Drives me nuts when I get that coin signal in one direction, and it is tough to find again. Do the 360 dance and then give up. May start digging some of those.