From A Wiki of Ice and Fire
The Fourteen Flames is an immense chain of volcanoes extending across the neck of the Valyrian peninsula. There were deep mines beneath the Fourteen Flames. The Fourteen Flames lit the Freehold’s nights of old.
As they grew in power the Valyrians sent their slaves to the mines beneath the Fourteen Flames, searching for gold among other things. Men of a hundred different nations laboured in those mines. The worst slaves were sent down to die in the red darkness of the mines.
The mines of old Valyria were always hot and they grew hotter as the shafts were driven deeper and deeper. The slaves toiled in an oven, the rocks around them were too hot to touch. The air stank of brimstone and would sear the slave’s lungs as they breathed it. Sometimes when they broke through a wall in search of gold, they would find steam instead, or boiling water or molten rock. Certain shafts were cut so low that the slaves could not stand upright so had to crawl or bend.
Sometimes firewyrms were encountered in that red darkness, in the shafts of the mines by the slaves, leaving only burnt and blackened corpses, yet still the mines drove deeper. Slaves perished by the score, but their masters did not care.
After the Doom
The Fourteen Flames appear to still be active in some way, even after the Doom. It is said that any sailor who so much as glimpses the fiery mountains of Valyria rising above the waves will soon die a dreadful death. 
|“||Fourteen or fourteen thousand. What man dares count them? It is not wise for mortals to look too deeply at those fires, my friend. Those are the fires of god’s own wrath, and no human flame can match them. We are small creatures, men.||”|