What do they think they will find? DNA? So what? A suicide note? A fake confession - "I, Joey Three Fingers Burelli confess that I hit Jimmy over the head with a shovel and buried him in a shallow grave in this field, like Don Corleone told me to? By now, old Jimmy is a hank of hair, a pile of bones and an unsavory memory. The FBI has searched longer and harder even than the Hoffa family, who gave up a long time ago, specifically in 1982, when he was declared legally dead and they inherited his estate.
Isn't this all just a bit silly? It has become an expensive obsession with the FBI. Why doesn't somebody invent a board game called "What Became of Jimmy?" All the agents who think they just have to know where Hoffa is can get together on a designated game night and play the game until the urge goes away.
My advice to the FBI? Stop. Just stop looking. As Judge Judy would say, "Put a period and move on." Everybody else has.