Indeed there are caches buried at depths around twenty feet. Never under estimate the limit of human endeavors. Confederate military teams were rather adept at digging and tunneling. Gold/treasure caches had to be buried so deeply because Union troops and others would use long metal rods to probe the ground searching for buried caches, etc.
An affordable option for locating deeply buried treasure caches and such would be a two-box unit (Treasure Hound) from Garrett. Very easy to use, but doubtful it would reach down to twenty feet unless the object(treasure cache) was rather large/automobile sized.
The best method, albeit rather expensive, would be ground penetrating radar or large blanket type towable pulse induction units.
Here are a couple of useful links to these more expensive but effective units.
http://www.okmmetaldetectors.com/products/professional-detectors.php?lang=en
http://www. a c c u r a t e l o c a t o r s . c o m / pulse_induction.html
Best of luck to ya!
Happy Hunting!
P.S.
If I were you and I found this gold cache, I wouldn't advertise it