I have a house that has 3/4" thick existing hardwood floor in hallway approx. 6' wide and 20' long with hardwood running length of hall. The center of hallway floor is approx. 3/4"to1" higher than the floor along the walls. As if it is arched in the center of hall. Also, walking on the floor feels extremely solid and doesn't give slightly with pressure. I know hardwood is solid, but other areas of the house (with same floor)has a softer feel than the corridor. I was thinking there wasn't an expansion gap along the walls and possibly this was making the floor arch. I removed one of the baseboards and it appears to have little to no expansion gap. I read that without an expansion gap, the floor could become wavey. However, my floor isn't wavey but rather arched in center. Has anyone seen similar? Could I just cut in an expansion gap and replace the baseboard?

If you do not have any expansion gap,there may be enough pressure from the wood flooring expanding to buckle up the center rows. Cut enough of the flooring so that the base board just covers the flooring with a toe kick saw. You can get one at Harbor Freight. If both sides of the hall are glued down, for a starter row and to finish without a lot of top-nails on the last few rows, you may not be successful even if you cut an expansion gap for the floor to move enough to settle down.
The flooring may go down over time by itself. If it has come off the sub-floor as much as you think it has, it will be loose.

Did this arch just happen? You say floors have a softer feel in other areas....what type of floor/installation is this? Is this a nail down floor?

Thanks Pete, I don't think the ends are glues down, but I will look for nail holes.

Jimmie,
No the arch has always been there since I purchased the house. The flooring is 3/4" thick oak. I understand oak has a hard feel normally, but this feel is much harder than other areas of the house with the same flooring. Hard to describe, but it's obvious when walking that the floor has less of a bounce in the hall than other areas.

If there is a well supported sub-floor there really shouldn't be a noticeable bounce when a solid wood floor is walked on. Also, an expansion gap at the edge of the flooring is there to allow the outer few rows to expand without exerting pressure on the walls, pipes, etc. If there is extreme expansion at the center of a floor those rows will press against each other and buckle upward. Is your arched area on the first floor above a basement or crawl space and can you check it from below to see if there is a severely crowned joist or something else that is causing the arch?

There is NOT a bounce on either floors. To put it into perspective I used the term bounce. The floor is obviously much much harder than the rest of the floor. Probably do to it pressing on the walls and arching up. The entire floor is on 2nd floor, you can close your eyes and walk from one area into hallway and you would notice the floor feels much harder. Not saying the other floor (non-hallway) areas are bouncy, but less hard while walking. Just as if you have hardwood on concrete as opposed to subfloor, the firmness will be different.
Also, I cannot access the hallway from below with out tearing out drywall but I would only see the subfloor anyway.

Does the arched area run along the centerline of the house?

Yes

With a house over about 20 feet wide the will usually be a beam in the middle of the house under the intersection of the floor joists. If the outside walls settle over time there can be a crowning where the joists intersect over the beam. If this is the problem, there should have been more preparation work to get the sub-floor flat before the flooring was installed.

Is there a wall on the first floor that is in line with the arch on the second floor?
Can you post some pictures? Sounds like an interesting situation. Any cracked walls? Any other floor areas sloped?