For a house built in 1993, something is definitely wrong if you have a 1" dip already. If it were me, I would start opening things up to determine the root cause of the sag.

Is there access from below or this is a slab?

If there is access from below and this is framed floor, check and see what caused the dip... It could be anything, a few not-crowned or undersized floor joists, etc...

Just inspected a job a week ago, they had the same issue, it turned out the previous owner removed a lolly column in the mid-span and the entire floor sank in the middle of the house.

If this is a slab, it could be a low spot in the concrete floor...

In any case hard to say without seeing.

Its the second floor, so cant see it from below because the first floor ceiling is finished. From the comments it seems like the best course is to take off the subfloor to see what the issue is. Thanks for responding!

Thanks! I checked the first floor ceiling and it does have a similar bulge. I take it this means some remediation on the floor joists is needed?

Such a large sag area for the room I would next start looking at the span and joist size. Check which way the floor joists run (subfloor orientation and nail pattern). 18' could be pushing it. Also evaluate if there could be plumbing across there where a plumber would have bored/notched the joists. Evaluate if there ever could have been a wall downstairs that was removed by a previous owner.

I took off a piece of the subfloor along the lowest point. The joist looked good - no rotting or anything - and it was actually a double joist. Theres no movement when you apply heavy weight or impact. Only thing I can think is the joists may have been put in wrong side up and/or someone had something really heavy in the room like a waterbed. But it seems structurally ok from what I can see. Anything else anyone thinks I should look out for?

How deep (high) was the joist you saw, what is the span of the joists, and what is the joist spacing?

The joist was at least 2x10 and maybe 2x12, spaced at 16”. Span would be about 15 feet. Everything looked pretty normal with no obvious issues and there was no movement under weight or impact.

On the advice of our inspector and a neighbor who’s a retired remodeler, we decided to put varying thickness plywood down over the gaps up to 1/4” and will self-level the rest. Hopefully no future issues. I don’t love the solution but seemed like the most practical approach. Thanks to everyone on this board for your advice!

Are you absolutely sure, a wall hasnt been removed on the lower floor? I would not leave it like that. I wouldnt want a bow in the ceiling below it.