With an old Jacto metal detector I bought at a garage sale for$10,I dug up a 5 gallon metal bucket in my front yard ,it was about a foot under the ground.
If it's more than a foot or so by a foot or so, And I can see old iron, I won't dig it. But I did hear a story in the "Treasure in the news" forum a long way back about someone finding a WW2 tank with there detector. I think that would take the cake!!