A mother dragon lays her eggs under the mountain where she was born, along with all her sisters. If the mountain is not large enough she and her mate must dig a new cave. This is very dangerous because during the 600 years underground, the dragon eggs grow from about 10 feet to 1000 feet long and run the risk of cracking. Eggs shells are made of many layers of a hard substance and develop to be 10 feet thick. The growth of the eggs is how mountains rise.