Undergrounds are optional PvE locations, which require perfectly tuned teams with powerful items as rewards and tokens which can be exchanged for an exotic weapon or armor. They're also one of the stage to craft the legendary weapon.
At this moment there are eight undergrounds in the game:
Story mode level
Exploration mode level
Plains of Ashford
Shire of Beetletun
Citadel of Flame
Honor of the Waves
Crucible of Eternity
Infinite Coil Reactor
Before going on, you can choose between two game modes: story mode and exploration mode. They are briefly described in the following paragraphs.
Only available mode at the first attempt to the underground. Some NPCs will join your team and tell about the history of Destiny's Edge. Difficulty level isn't high in this case and even playing with the new friends you can achieve a success. Completing the story mode unlocks an access to the exploration mode.
It's the main mode of the game, where you collect tokens and can search all, previously unavailable, parts of the dungeon. It's much more difficult than the previous one and requires better teamplay, proper skills and equipment. In this mode the team may decide which quests you should complete (there is a vote, the majority decides) and run into typical random events like cave troll attack. Thanks to the randomness of events and few available paths you can replay every underground, getting a new adventure each time.
It's worth be mentioning that monsters in the dungeon don't respawn and the new waypoints are periodically unlocked. Team members can be changed any time, and those who didn't complete the story mode may take part in the exploration mode if they're invited by the player who did.
As I mentioned, the most important motivation of going to the undergrounds are rewards. During the exploration you find not only loot from defeated enemies and chests full of treasures, but also various tokens, depending on the underground you're in. They can be exchanged at traders in Lion's Arch in the Fort Mariner for rare or exotic items related to the particular dungeon. So while exploring the Citadel of Flame you get mostly equipment connected with the fire: armors with Baelfire rune or flaming swords and axes. But remember, that the weapon you can buy, won't be more powerful from the one you can craft or find in the other location, their main value is an unique appearance.
Undergrounds are perfect for increasing your ability in fighting with the most powerful enemies in the PvE and also get tones of items for trade and dismantle. What is more, tokens encourage to complete undergrounds many times: everyone wants to look uniquely.