As mathematicians continue to study mathematics, often times they run into a problem which takes a considerable amount of effort to solve. For instance, trying to factor polynomials has lead to a large portion of group theory and Galois theory especially. Another example, which I have heard reference from one of my professors, is that the study of elliptic functions has lead to a large amount of Algebraic Geometry. The original goal being how to integrate elliptic curves turned into quite an interesting field over the years.
My question, although "broad", is this:
What are some mathematical problems that have been forgotten?
In the process of trying to solve one question, often times many more questions are discovered. I find it hard to believe that no question has been forgotten. As in, sometimes the cutting-edge work of mathematics has trumped the questions that seemed of lesser importance at the time of research, and after a while most everyone has forgotten about them. Surely some of us must remember! Or, are there any stories of such problems like this that have sparked new mathematical research over the years?