skold wrote:unless you bury it in 6ft of dirt :p
By the time the hole is filled, the safe will be caved in.
I'd say 1/4 of an inch thick is good if you want absoluted authenticity, but 1/8 or 1/16 of an inch is fine (sorry, forgot what guage that is). What I'd do if I were making a "vault" type door is find a 3-point mortise lockset and sandwich it inbetween an 1/8 inch thick steel plates.

"Lockpicking is what robbing is all about!" says Jim King.