Well.....
Sounds to me like you dont even have a plywood subfloor, generally what will happen if subflooring gets wet
Particle board: wont hump up, just gets mushy and eventually falls through.
Plywood: Generally any plywood used for subflooring will be Exterior grade plywood (CDX, BCX etc.), meaning that it can stand some amount of water on it. Remember that when the house was built, the subfloor was exposed to the elements while under construction. Plywood that is not exterior grade and exposed to water will generally just delaminate, and feel slightly mushy and sqeaky underfoot, not hump up.
1x8 fir (or similar) wood strips used for subflooring will cup (the board will get a U shape to it) if wet, causing some humping, but smaller humps and more of them.
It sounds like the winner is:
Oak tongue and groove flooring, covered by carpet. Oak flooring will hump badly if exposed to water, and needless to say, it is the most expensive to repair by far, since it has to be done by a professional flooring company to make it turn out right. It has large humps since the boards are tied together by the tongue and groove feature, which also makes it really hard to work on.
I hope you have plywood, but it doesnt sound good, especially if it is a slightly older house.