Underworld

The 'Underworld (闇) is the whole system of connections, smugglings and actions which eludes the World Government's control. It's mainly set in the New World.

Organization
According to Law, the New World is divided in territories governed by the strongest pirates thanks to their brokers, pretty similar to a huge crime syndicate. The most influent man in this world is Donquixote Doflamingo via his Joker alias.

Smuggling
One of the main scopes where the Underworld acts is the smuggling of illegal goods via Brokers.

Past
In the last ten years, Donquixote Doflamingo built up his status as the most influent man in the Underworld under his alias of Joker.

Baroque Works Saga
Crocodile claims to have obteined the Dance Powder via smuggling. This will cause the Alubarna's civil war.

CP9 Saga
Franky claims to have bought the rare Adam Wood in the St. Poplar's black market.

Whitebeard War Saga
Before reaches the Sabaody Archipelago, the Straw Hat Pirates defeated Duval's Tobiuo Riders, one of the best kidnapping group and slaves supplier of Disco's Human Auctioning House.

During the Straw Hat Pirates' visit to the Sabaody Park the Hound Pets captured Kaimy and sold her to Disco. This causes Luffy's rage, his attack to the Auctioning House and the whole Roswald's Accident. Due to this, Doflamingo quits with Disco and gives him the entire House.

During the Timeskip
Due to Doflamingo's abandonment, the Human Auction fails.

Trivia

 * When Donquixote Doflamingo indicated Kuzan as an Underworld member, he not answer, leaving pending if he is or not.