Molten Core

The Molten Core is a level 60 raid dungeon located within Blackrock Mountain, home to Ragnaros the Firelord. It was one of two forty (40) player raid dungeons available upon release of World of Warcraft, the other beingOnyxia's Lair. Considered the pinnacle of raiding achievement at the time, the Molten Core enticed adventurers to enter the instance in the search of realm glory.

History
War of the Three Hammers.

The Molten Core was created during the War of the Three Hammers more than 300 years ago. Near the end of the war, Thaurissan, the leader of the Dark Iron Dwarves, sought to summon a powerful fire elemental to defeat the combined forces of the Bronzebeard and Wildhammer clans. He was more successful than he could have imagined, as he released Ragnaros the Firelord from millennia of captivity under the Redridge Mountains.

Ragnaros destroyed the city of Thaurissan and created the volcano of Blackrock Mountain. He dwells there to this day with his elemental servants and the enslaved remnants of the Dark Iron dwarf clan. The burning lake where Ragnaros lies sleeping acts as a rift connecting to the plane of fire, allowing the malicious elementals to pass through.

From Blizzard's Official World Dungeons site:
 * The Molten Core lies at the very bottom of Blackrock Depths. It is the heart of Blackrock Mountain and the exact spot where, long ago in a desperate bid to turn the tide of the dwarven civil war, Emperor Thaurissan summoned the elemental Firelord, Ragnaros, into the world. Though the firelord is incapable of straying far from the blazing Core, it is believed that his elemental minions command the Dark Iron dwarves, who are in the midst of creating armies out of living stone. The burning lake where Ragnaros lies sleeping acts as a rift connecting to the plane of fire, allowing the malicious elementals to pass through. Chief among Ragnaros' agents isMajordomo Executus - for this cunning elemental is the only one capable of calling the Firelord from his slumber.

Geography
The Molten Core in the TCG.

The Molten Core consists of rock and lava, with the lava forming rivers, small lakes and "lavafalls". Other features are small volcano-like smoke vents, molten cracks in the walls and roof, occasional dark iron deposits, and the runes of warding. Most locations do not have specific names, causing the majority of the Molten Core to be called simply "The Molten Core".