The Cave is added once you buy the Hatch. It starts off with 4 spaces with Firefly Hemps ready to be harvested. For 500,000/space it can be expanded to 24 spaces like the Field and Island. Firefly Hemp Yield 7 Hemp in the cave instead of 1 if it was grown anywhere else.
If pawned the Cave area disappears and when bought again it resets everything in the Cave.(All your plants Hemp, Stuff, and Incressed tiles disappears and get reset to 4 planting tiles with 4 firefly Hemp ready to be harvested.)