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.

Lucifron
Magmadar
Gehennas
Garr
Baron Geddon
Shazzrah
Sulfuron
Golemagg
Major Domo
Ragnaros