This is the skull of Enceladus, the giant who fought with the Greek gods of Olympus. In mythology, this is written about him:

Enceladus (like other vanquished monsters, thought to be buried under volcanoes) was said to be the cause of earthquakes and volcanic eruptions. Mount Etna's eruptions were said to be the breath of Enceladus, and its tremors to be caused by him rolling over from side to side beneath the mountain. So, for example Virgil:

Enceladus, his body lightning-scarred,
lies prisoned under all, so runs the tale:
o'er him gigantic Aetna breathes in fire
from crack and seam; and if he haply turn
to change his wearied side, Trinacria's isle
trembles and moans, and thick fumes mantle heaven