The Mine

After you beat floor 500. You can purchase 3 miners to work on a mine to starting farming gold and rare materials.

Here is what it looks like when you enter the mine.

You can invest in 3 different miners in this game.

The mine is located across the blue tent near World Dungeon Boss 3. They all cost green diamonds to start. Afterwards you can upgrade them with gold to get a steady return of gold even if you are offline. The best miner to upgrade to Level 50 is miner 3. It gets complicated after that. The more important thing is that the miners get you rare ores and materials you need to upgrade your weapons/armor/etc past Rank S + 30. The return on investment for gold is not great at first but if you play the game long term its worth pumping a level per day. So think of the mine as an investment in those ores.

When you get back online, go to the mine and click on the box on the right of the screen and you will collect all the gold that was collected when you were offline.

Here is a link to an uncompleted Mine spreadsheet for the public wiki. You can copy it and make your own or add to the spreadsheet.