Does anyone have a list of decent sites which include a range of either unsolved mathematical problems, or unproven formulae? There used to be on liked from my employer's mathematical index, but it is no longer there. I understand that the possibility of doing something like finding all the zeros of the riemann zeta function is impossible for most of the world's population(including me), but regardless, I am curious as to where the mathematics train consistently runs into the wall. I'm mostly interested in the history of theory that went unproven for a long time and what it took (both time and ingenius mathematical concepts) to solve or prove these theories. Thanks.