Treasure hunting with a compass

[bigstemz]

ok, so im sorta speaking out of my but here because i know nothing about how to do this but...

...yesterday i was watching some treasure hunters on tv look for jesse james hidden treasure cache, they were using a compass at at certian areas it would deviate from north by up to 30 degree. then when they hit that area with ground penetrating radar they found anamolies under the ground that may or may not be a cache. does anyone know how the compass deviates like that or how a site that better describes this sort of compass technique?