Cave will be unlocked when you find the last part of the Mysterious diagram and build the City Gate.
Cave is one of the unique buildings in the game such as Ship and Pyramid. When you unlock Cave you will access unique match-3 levels and modes.
To restore the Cave find necessary collection items. Complete collections to get these items. To build the Cave you need resources. Building takes time.
When the Cave is ready you will open new levels in Treasure Hunt mode and find Skystones.
Each set of ruins is a match-3 levels with unique tasks. You need resources to play a match-3 levels. Complete tasks to get shovels you need to upgrade the Cave.
It takes time to excavate from one set of ruins to another.
There are 7 available levels of the Cave and 125 match-3 levels to beat. To upgrade the Cave you need to fulfill the requirements. You also need resources to upgrade the Cave. Upgrading takes time but you can speed it up using your crystals.