Yeah, I was thinking maybe it was a large hearth that had wings to the side. I don't know that the hearthstone on the floor would cause that sag though.

I second /u/Spinaccio's approach for putting a post in. Or posts - I'm having some trouble envisioning how exactly the fireplace is supported by that (where is the ash pit?)

Another option to explore with an engineer... In our house (1931), I've got full access to the exterior basement wall "top plate" (concrete) and the interior wall top plate. If I had this problem under my fireplace, I would just raise the floor a bit temporarily with a jack and slide in at least two brand new joists. I don't know that you would want to sister these to the old joists, though.

But as for your dip... really depends. I've noticed a lot of uneven spots in my floors, but no damage to the joists underneath. What I suspect is happening is 100 years of uneven wear and settling and compression in the softer wood subfloor. If it moves, you could also have the classic subfloor board that ends just shy of a joist.

If you know the approximate time and location of the photo, and you have a helpful celestial reference like the moon in your photo, you can use this website, (which is amazing for many other reasons) to determine the direction of the photo.

With that and the original location, you should be able to determine the peak.

At least as owned by Cousteau... it was already the Ship of Jaqueseus before it ever left port under his sail.