Hi - I moved into my house about a year ago. The main floor has 2-1/4 inch red oak flooring that I assume was installed when the house was built in 1985. In the past week, I noticed cupping in a localized area that's about 3ft x 6ft (it extends further on 1 or 2 planks). The cupping isn't very noticeable from looking at it, but you can definitely feel it dragging your foot across the planks. It's winter in Chicago, and the air is pretty dry. The humidity on the first floor today has been between 30-34% and the basement is essentially the same (+/- 1 or 2%). The area of the cupping is in the living room about 4-5 feet from front door, and it's not anywhere near any appliances or plumbing fixtures.

I'm trying to figure out what is the probable cause of the cupping. Could it be from someone dragging in some snow that melted on the floor? Also, what's the best course of action to fix the issue? This is my first post, so thank you in advance for your help!

The cupping is from moisture and it may travel under the planks. Is the front door in the weather? Do you have a storm door outside your front door? How does the threshold look?
If the flooring runs toward the front door, this would be first to check.

Thanks for your reply, Pete! There's no storm door, but the first level is about 5-6 feet above ground. While the flooring runs toward the front door, the cupped planks are not adjacent to the door (see photo: https://i.imgur.com/60rb17d.png). In fact, the cupping is occurring towards the middle of the room. I'm really puzzled as to what could be the source of the moisture. We are hardly even in that room (since we mostly enter and exit through a side door). Is it plausible that someone coming in from the snow outside and say, dropping their snow covered winter coat on the floor could have caused this? I obviously would really like to identify the source of the moisture to make sure I can have the underlying problem fixed. Thanks so much for your help!

No chance of a plumbing leak? That is some serious cupping. Is the ceiling finished in the basement? There may be a pipe that travels under the floor. A small puddle would not penetrate through the finish like what has happened here. A non-invasive moisture meter could help determining where the water comes from.

I have no expertise on this stuff, so I really appreciate your help, Pete. I'm stumped what the cause could be. The ceiling is finished in the basement, but there is closet with a drop ceiling that allows me to see the joists and subfloor pretty close to the area of the cupping. Maybe I don't know what I should be looking for, but I don't see any evidence of a leak and I also don't feel moisture. Here are some photos from that closet: https://imgur.com/a/k0RfvaI.

Thanks again for your help. I was planning on calling the contractor who installed hardwood floors for us upstairs once the wood had time to dry, but should I be calling a plumber first?

A contractor with a moisture meter could be of help.

Thanks, Pete. The contractor who installed hardwood floors in my upstairs is coming by tomorrow to check it out. Let's hope he can solve the mystery.