The Mines of Thunderstone Keep

Deep within the Mines of Thunderstone Keep lie the skeletal remains of the Clan of Yigal Thunderstone, the mighty Dwarf chieftain that led the Thunderstone Clan to control Caer Duna in 231 AF. When Chieftain Fenzen Swinghammer launched his own attack of the castle in 234 AF, Chieftain Thunderstone and his people were pushed back to Tonnta-a-tonna where they laid down roots and dug deep into the earth in search of riches and precious metals for smithing.
By 341 AF, Clan Thunderstone had reached what they believed to be the foundation of the earth. The clan, now led by Chieftain Yegindal Thunderstone, was fueled by their lust for riches and insisted that they dig deeper and deeper, even employing the explosive knowledge of Sven Boombeard, the best explosives engineer in all of Camdara, to blast their way through the thick layer of impenetrable earth.
Sven crafted over three hundred barrels of dwarven fire to use as part of a controlled explosion into the center of the earth below, but unfortunately, things did not go according to plan. The Thunderstone Clan did not properly seal up the chamber before igniting the dwarven fire and the explosion sent a pillar of fire straight up, destroying the complex system of supports that the Dwarves had and collapsing the entire cave network in on itself, trapping all those below to a painful, agonizing, lonely death.
Now, over 100 years later, the cave has still not been touched since that day and even the people at Tonnta-a-Tonna refuse to go down the mineshaft, much less peel back the overgrown plant life that covers it.