Alright, let's say that I find a gold mine large enough that if mined and refined, all the gold in it were enough to make gold about as rare as dirt.
First, what effect would this have on the economy? I heard somewhere that the wealth of nations is based on how much gold each country has.
Second, what if I could mine and sell it, like, really super fast without anyone knowing? Could I hypothetically become rich before anyone realized what was happening?
The effect would be inflation.
In the pre-bretton woods world, money was gold. So more gold = more money in circulation. Now money is not gold, so, theoretically, even if gold was 100x it wouldn't matter.
BUT, since the monetary supply of fiat currencies is inflated enormously by those who own the keys to the printing press, the only thing that constrains them is the price of tangible assets that would reflect their printing activities. Gold price spiking is an indication of people losing faith in the value of fiat.
More gold would allow lower gold prices and as a consequence an expansion of "printing" activities as people would not be able to tell the difference since the gold price would decline.
The equivalent of the gold mine you describe is "paper gold". The invention of the bankers to multiply the quantities of gold traded, like it was actual gold, when in fact it is not. There is an artificial "supply" of gold (as much as 100x compared to actual gold) which keeps gold price down and the printing press going.
Their leverage between physical and fictional gold is only constrained by the lack of actual metal. Multiplying physical gold by, say, 10 times, due to some discovery, would allow them to leverage paper gold much higher (like 1000x) because there wouldn't be physical shortages of any kind and thus the situation would be well under control.