The snake comes out as through "from the burning bowels of the earth /On the day of Sicilian July, with Etna smoking."  Lawrence is imagining that the snake is descended from the mythological serpent Typhon.  Here is The myth of the volcano Mt. Aetna and Typhon:
Typhon was born from Gaia (Mother Earth) and Tartarus. This was Gaia's youngest offspring, but by far the deadliest and the largest monster ever conceived. Its thighs were gigantic coiled serpents; its arms could spread across the heavens, and its head (in the shape of an ass's head) touched the stars. When it took flight, its wings blotted out the sun, and when it opened its mouth, out came burning boulders. Typhon was so frightful even the gods of Olympus refused to fight, fleeing instead to Egypt when Typhon attacked their mountain home. Each god disguised itself into an animal: Zeus transformed himself into a ram, Dionysus a goat, and so on. Aphrodite and Eros both disguised themselves as fish and swam up the Nile to escape the monster. Typhon was eventually defeated, due in large part to the brave and level-headed Athene, who convinced Zeus to take up his thunderbolts and make battle. Typhon actually captured Zeus and placed him in a cave, but Hermes and Pan were able to free him. To make a long story short, Zeus then took the battle to Typhon, chasing him to Sicily. There Zeus threw Mount Aetna at the monster, finally subduing it. But under the earth, the buried monster still spews up fire and boulders every so often.