The 1874 dime is the deepest coin I have ever dug.  Going by the lack of wear on the coin it was probably lost in the 1870's.  It was found in the middle of an old ball field.  The VDI reading on my White's XLT indicated a solid 60 but I could get a 70/71 Penny/Dime reading if I slowed the coil down.  When I went to pin point, the depth indicated was 7 inches.  It was every bit of that.  Another Penny/Dime reading resulted in a nice, but unmarked toy pistol.