1: would be the grave that can reveal a coffin that can be opend to but a dead body in. the dead body can ofcourse have things stored on it. The grave mound that would be expoxed to the views of the landscape would have any number of diffrent looks and would be the trigger to dig up the coffin. it can have crosses and stuff as head stones and what ever.
2: An smaller hole that would take less time to dig up would simply have a chest in it like any chest in the game. But you would get to choose what kind of mound you want like a rock mound or a simple dirt mound. The treasuer chests and graves would be marked on your map where you barried the stuff. now i don't expext it to be scripted to be able to barrie anywhere. but here nad there where ever the desighner wanted unless it whould be just as easy to make it to where the player could dig everywhere, but I dout it.
I'm sure anynumber of ways could be implamented to make this possible....
