I mean if you want to be maximally efficient about it just find it abandoned open pit mine in the middle of nowhere. You could dump all of the trash and corpses the human race has ever produced in it and barely even scratch the surface. Just one problem.
How the fuck are you going to pay for all the transportation expenses?
I often wondered about that. Humans have been around for 100,000 years or so and we've been putting people in graves for a much smaller portion of that, but it seems we would have an awful lot of graves at this point. But we don't. You don't come across cemeteries very often and when you do they're not enormous.
My great grandfather’s grave is still around and he died in the ‘80s, in a cemetery in a highly populated part of my city. Right next to him is his son that was killed in Vietnam in the ‘60s.
Depends on region of course, but I think most graves are around for much longer than 6 years.