The Underbelly

The Underbelly is an underground part of Dalaran. You can PvP, pet-battle, fish, buy high rank profession recipes there. There is also an achievement that grants you [item=138387] mount - Underbelly Tycoon. A lot of quests send you into the Underbelly so you better be familiar with how things work or you'll be ganked.
There are three entrances into The Underbelly - sewers near Krosus Landing and The Violet Citadel, there is also a well near the bank which teleports you into random location in sewers.
Zone has it's own currency - [currency=1149]. Almost every activity in underbelly is connected with this currency. You can spend it on a lot of useful things from pet battles to different high rank profession recipes.

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How to get [currency=1149]:

1. 
   
2. PvP
   When you enter Underbelly you get [spell=192229] or [spell=192221] debuff. [spell=192229] means the guards are on their posts and no PvP is allowed. [spell=192221] means there is no guards and Underbelly becomes free for all PvP zone. During that time you can farm Sightless Eyes by killing other players, you get from 15 to 50 Eyes for each kill.
   Guard Captain [npc=97359] is the one controlling the guards. You can bribe him to call off guards or call them back for 50 Sightless Eyes. You can even hire personal guard if guards are not on their posts for 5 gold and you'll not be flagged for PvP.
3. 
   
4. Mobs
   Some vendors in the Underbelly sell items for [currency=1149] that can be used to summon different mobs around the area. Anyone can engage in combat with them but you'll get the loot anyway if you tagged the mob, you get about 1-10 eyes for each kill.
5. 
   
   
6. Bosses
   There are mini-bosses that will spawn when you have [spell=192229] debuff which means the guards are on their posts and no PvP is allowed. You'll get 50-90 eyes for each boss.
   You can double the number of eyes everyone gets from bosses by using [item=139427] on them, you have to use it when the boss has more than 90% health. It increases the health of the boss by 200% and damage by 50%.
   All mini-bosses scale their health based on the number of players attacking them.
   
   Boss

   Note

   Video

   [npc=97587]

   Fire mage, don't stand in aoe

   https://www.youtube.com/watch?v=XzKsMZuKdPA

   [npc=97390]

   walking in stealth

   https://www.youtube.com/watch?v=Hy9nYze2P78

   [npc=97388]

   A demon near warlock class hall

   https://www.youtube.com/watch?v=n9rf8gVbFnU

   [npc=97380]

   Giant rat in the main hall

   https://www.youtube.com/watch?v=WfL0uWP9oWE

   [npc=97387]

   Mana fiend from the sewer pipe

   https://www.youtube.com/watch?v=woF8LNE_14A

   [npc=97381]

   Giant bat in the main hall

   https://www.youtube.com/watch?v=jZycU6BaWA8

   [npc=97384]

   Shark in Black Market waters

   https://www.youtube.com/watch?v=TEYGinl1rAY

   [npc=97589]

   Spawns lots of spiders

   https://www.youtube.com/watch?v=Ia45srdb0_E
   
   
   
   
   
7. 
   
   
8. Chests
   There are three types (small, medium, large) of chests that will spawn when guards are not on their posts - [spell=192221]. The bigger the chest the more time you need to open it. You have to be flagged for PvP in order to loot the chest. Small chest gives 10 eyes, medium 40 and large 100.
   
9. 
   
10. Fishing
    Fish pools will spawn in the Underbelly Black Market Auction House area. Your fishing skill doesn't matter. You can create fish pool by using [item=139422] which is sold by [npc=97361] for 350 [item=139422]. Caught fish [item=139416] contains 4-5 eyes.
11. 
    
    
12. Quests
    [npc=110642] is in the Alchemist Lab The Underbelly at 75.8, 83.2 (south-east on the map),
    she offers you Experimental Potion: Test Subjects Needed which will transform you into a
    various creatures granting different Experimental Potion buffs for 20 minutes.
    Buff persists through death and is working only in The Underbelly.
    Quest reward is 150 [currency=1149].
    You will be transformed into one of the following creatures:
    
    • 
      
    • Skrog
      Transform you into a skrog when you are in a space that is large enough to contain one.
      While transformed, your health and damage are tripled.
      https://www.youtube.com/watch?v=h_7Xguo7ulw
      
    • 
      
    • Nightfallen
      For casters receiving fatal damage will instead transform you into a withered.
      For melee transformed into a nightborne. Haste increased by 10%.
      https://www.youtube.com/watch?v=tTjtv1PU1Hw
    • 
      
    • Tuskarr
      You can fish Sightless Eyes from open water in the Underbelly.
      https://www.youtube.com/watch?v=DojJayxcCk8
      
    • 
      
    • Kobold
      You can collect candles from the corpses of your enemies in the Dalaran Underbelly.
      
    • 
      
    • Harpy
      While within The Underbelly, you can fly for short periods of time.
      https://www.youtube.com/watch?v=gMJ-LjfIdLg
    • 
      
    • Dryad
      You can use the [spell=219734] ability.
      
    • 
      
13. 
    

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How to spend [currency=1149]:

1. 
   
2. Summon Mobs
   You can buy items from vendors around the Underbelly which summon various mobs. Buff bosses with [item=139427].
   
   Item

   Vendor

   [currency=1149]

   Spawns

   Video

   [item=139423]

   [npc=97363]

   175

   [npc=110719]

   https://www.youtube.com/watch?v=RP3sA1Tovnk

   [item=139425]

   [npc=97360]

   225

   [npc=110721]

   https://www.youtube.com/watch?v=C9XiKheheRg

   [item=139424]

   [npc=107760]

   175

   [npc=110729]

   https://www.youtube.com/watch?v=wWK4cBLpaCo

   [item=139426]

   [npc=97366]

   250

   [npc=110737]

   https://www.youtube.com/watch?v=gaEhUlFjSu4

   [item=139427]

   [npc=97362]

   175

   Buffs mini-boss

   https://www.youtube.com/watch?v=vgXj2tbIsBA
   
   
3. 
   
4. Professions
   A lot of profession recipes can be bought from vendors.
5. 
   
6. Fun items
   [item=139412] from [npc=97364] costs 1500 [currency=1149], places 4 [npc=110666] that can be fought and captured in a pet fight.
   [item=136904] from [npc=97364] costs 150 [currency=1149]. A battle pet that can be sold on AH.
   [item=139431] from [npc=97359] costs 950 [currency=1149]. Creates a cage near Raethan from which the first 25 users can loot [item=139421], which allows them to ride Ratstallion.
   [item=139411] from [npc=107764] costs 650 [currency=1149]. Places a banquet outside the door for everyone, grant 150 intellect after eating.
7. 
   

Vendors location:

Name

Coordinates

Video

[npc=97366]

71.7, 24.0

https://www.youtube.com/watch?v=Ss587e-jPYY

[npc=97360]

46.95, 56.15

https://www.youtube.com/watch?v=CrFtuKa_XP0

[npc=97363]

65.7, 79.9

https://www.youtube.com/watch?v=BKR_VIEHykA

[npc=107760]

65.9, 80.9

https://www.youtube.com/watch?v=TqGAVPujJRc

[npc=97364]

58.1, 58.0

https://www.youtube.com/watch?v=T12deCFzm6Y

[npc=97361]

67.0, 17.0

https://www.youtube.com/watch?v=KRIav_RUSFM

[npc=97359]

68.8, 58.2

https://www.youtube.com/watch?v=ed_FqfdKZNg

[npc=106887]

50.9, 38.5 (rare spawn)

https://www.youtube.com/watch?v=A_KvY_isaIc

[npc=107764]

62.45, 80.2

https://www.youtube.com/watch?v=96QdGbfqAX8

[item=138387] mount demonstration https://www.youtube.com/watch?v=db9zY4B3h68

When the guards in the Underbelly are bribed, they seem to be off duty for 5 mins.

This area is pretty ridiculous, If you are down here for anything other than PvP. you will die within 10 seconds of the guards going. When you get to the staging area after presumably dying, and the guards are still missing, you can buy a bodyguard for 5g from [npc=97359] which will stop you being PvP enabled.