I have fought various zombies in the Abysmal Mines...up to level 7 now, but I CANNOT work out how to..

Once entering the mining area, nine mines with green crystals are displayed. The mines may be occupied (picture on top of mine) or empty (no picture). You can gather crystals from an empty mine.

Of course, all nine mines may be occupied. There is a page number indicating 1. This is the first of 29888 pages of nine mines. You can use the arrows or write in a different number and hit enter to change page. There is also a quick find button to find a page with at least one empty mine.

Click on an empty mine. An occupy button and gathering time will display. Select occupy. Then send your forces to gather from the mine. The gathering time will start counting down. At the end of the countdown, the gathered crystals will appear in the upper right-hand corner and can be used in the shop