Molten Core, the first true step along the path to raiding in world of warcraft. Home to a large variety of fiery monsters such as the molten giant or the lava imp it is quite a dangerous place for newcomers.
Before attempting to venture further then half-way into the dungeon you will be required to have a large amount of fire resistance gear, un-prepared guilds will perish. Also, the fire lord, Ragnaros, has been known to stagnate the progression of many a guild that does not have the proper gear nor the coordination to face him.
From the World of Warcraft 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.