Was there an intact "bottom" to the drum, too?
My guess is the drum was used to stash something. That square opening on the top looks like a good place to stick a 2x4 to pry the top open quickly.
You might want to get a metal detector, and sweep the area for more metal. That "witches circle" was definitely used for something shady. If not moonshine, they may have been cooking meth or something like that. Keeping it overgrown and hidden would be a definite advantage.
What we are looking at was the "bottom".... it was not a removable lid. We cut through it with the sawzall. Whether the other end was sealed, I don't really know. I didn't stand in the bottom of it, but maybe I should have. Just looked like they took a barrel, either with a lid on it or not, and buried it upside down.
It would not surprise me at all if they were cooking meth back there... Although we didn't find a lot of that ~kind~ of garbage (the empty Actetone, propane tanks.... etc...) we have found more of this kind of crap...