So far the only solid evidence they've found is in archives & tells the most interesting story - the ex-slave who bought up plots of land & died a rich man. Some of the stuff found on those plots is also the most interesting. The rest is all conjecture, lore & conspiracy theories. No hard evidence of anything at all. Just timbers dug up from past failed attempts.
What did they find in the 1st or 2nd season? ONE gold coin down some shaft where others had already dug? I never saw those seasons but all the replays indicate that's the only thing they've found to date. (Correct me if I'm wrong.)
Even if they do find something big eventually, it can't make up for the time, anguish, death & money it's cost.
Obviously these are "journey" people. Not "destination" or they'd have given up long ago.
And same goes for us fools who watch. It's about the journey, not the destination at this point.