What's on the third story of Thornfield Hall?

After Jane saves Mr. Rochester's life from the fire, he tells Jane to stay in the room while he goes to the third floor of the house. After a considerable amount of time he returns down and the entire problem is solved. However, Mr. Rochester never actually mentions what's up there, all he does is blame the fire on Grace Poole. What happens on the third floor is a mystery to both the reader and to Jane Eyre. We know that Grace Poole goes up there with food and water every day. With that in mind, one has to wonder what's up there.

Personally, I have no idea. However, I do believe it has to have something to do with Grace Poole; she spends too much time up there for it to be a coincidence. Perhaps Grace and Mr. Rochester have, or previously had, some sort of special relationship where Grace is mostly kept out of the public eye but enjoys some sort of relationship with Mr. Rochester. Either way, for now it's a mystery to both the reader and to Jane Eyre.