Some resources can be found only in Caves. To collect them you'll need some Shovels - don't forget to bring them along! Each Cave also requires certain resources for exploring. Click on the Explore button when you're ready!
Here it is, a whole Cave for you to explore:
To clear the Cave you need to click on various objects in the Cave. Each item requires a Shovel - 1 Shovel per 1 object. You can exit the cave at any moment by clicking on the Exit button in the bottom left corner of the screen:
After you dig up all the items, the corresponding notification will pop-up: