"Stranger Things" Season 4 is leaking various secrets, including a big secret about Hopper that David Harbour and the Duffer brothers have known since Season 1.

"Stranger Things" Season 4 is still in production and has no release date. And Haber, who is promoting Marvel's "Black Widow," recently teased some juicy revelations about the "box in the attic" and the "really cool" secrets he has kept over the years.

When we last saw Hopper, he appeared to be dead and was later mourned by his adopted daughter Eleven (Millie Bobby Brown). However, in a post-credits scene, it was suggested that the sheriff is alive and is being held prisoner in Russia. A teaser trailer confirmed this.

In an interview with Entertainment Tonight (h/t SlashFilm), Haber teased Season 4 as "the heaviest drama yet."

"We're going to see a lot of Hopper at his most vulnerable, and a lot of backstory that was only hinted at in the boxes in the attic will be revealed.

"And the actor said...

The boxes he refers to first appeared in Season 2, Chapter 4, "Will the Wise." And indeed, they were in Hopper's basement. Eleven found a pile of boxes labeled "Dad," "New York," "Vietnam," and "Hawkins Lab." She peeked into the last box, but the others remained a mystery.

As the new season delves into Hopper's backstory, it will reveal what Harbour and the show's creators had been thinking all along in the beginning.

"There's something in this season that I knew from the first frame of the first season that had a very big role," he previewed. Me and Duffers, from the moment we started shooting, we were like, "Oh, this will be cool, right?" I guessed that it was going to be."

"I never thought we'd be able to do a second season, but now we can do it this season, and I had the idea five years ago and said, 'Oh, this is the season where I'm going to let you in on this secret that I used to use as a subtle way to let you know something. We're finally going to break it to you, which is really cool.'

Will everything be turned upside down? We'll just have to wait for "Stranger Things" Season 4 to be released.