Hollow Mountains

With the Caves and Cliffs Update on the horizon, I would love to see some ways which the mountains and caves work in tandem with each other. 

Introducing Hollow Mountains:

On the surface, Hollow Mountains would look and generate like ordinary ones, but what makes them interesting is below the ground. When you dig a few blocks beneath the structure's surface, you will see that the inside of the mountain is actually a huge cave, brimming with monsters and resources.

Why Add This?

Hollow Mountains would provide a unique and incredibly interesting caving experience, as well as also being a new, easy source of emeralds.