Deep under Dragonblight, is the Nerubian kingdom of Azjol-Nerub. The nerubians are a race of spider-like nightmare fuel that dwell underground.
They are technially neutral, but a large contingent of Nerubians found themselves allied with Arthas when one of their rulers, Anub'arak, who was biased, since he was technically an undead, having been killed and raised by the spirit of the Lich King in the years prior to Arthas' reign of terror, offered Arthas aid on his road to Icecrown. He took the prince underground through Azjol-Nerub, in an effort to get around the combined forces of Kael'Thas and Illidan, and saved his life on several occasions, by guiding him through the treacherous terrain. In WoW, you enter the two dungeon instances that make up Azjol-Nerub through a large sinkhole in Dragonblight.
The shorter, and easier instance, is Azjol-Nerub.
It's a relatively short instance, and a party can usually get through it in about 15-20 minutes. You pass through the Gilded Gate, guarded by one of Anub'arak's lieutenants....
and find yourself in Hadronox's Lair, where you do battle with a fierce giant spider.
After a quick plunge into a seemingly bottomless pit...
you splash down into the Passage of Lost Fiends, where you eventually come face-to-face with Anub'arak himself.
The other instance is Ahn'kahet: The Old Kingdom.
This instance is much more sprawling than Azjol-Nerub, and features more trash and bosses.
An underground swamp area brings some funky fungi to take care of....
on your way to the massive temple where the final boss, the Herald Volazj an agent of the evil Old Gods, like C'thun, who are spreading their dark influence through the world, waits.