Otogakure

was the personal hidden village of Orochimaru, which was founded for the express purpose of collecting ninja for his experiments and his quest to learn all techniques.

Background
Initially, Orochimaru claimed that he founded this village so that skilled ninja would find their purpose in life without being beholden to the fickle and warmongering priorities of the Five Great Shinobi Countries. In actuality, Otogakure is not really a village, but a group of hideouts and bases scattered throughout the Land of Sound and various other countries. Sound ninja use a diverse amount of techniques, which are mostly centred around the modifications and mutations Orochimaru gave them.

Team Dosu, under Orochimaru's orders, entered the Chūnin Exams with the goal of killing Sasuke Uchiha. However, Orochimaru was actually using them as test subjects for Sasuke's power after Orochimaru gave him the cursed seal. Later, Orochimaru tricked Sunagakure into helping Otogakure invade Konoha. Based on their map, Hidan and Kakuzu visited Otogakure while searching for jinchūriki.

Locations
Orochimaru set of many bases across the world during his travels. These based ranged from usually inconspicuous underground hideouts with holes for entrances to more grand ones hidden on an island. All these hideouts were outfitted with laboratories where he and later Kabuto Yakushi would carry out experiments.

Land of Sound Base
The former headquarters of Otogakure up to at least Chūnin Exams, the stronghold was built below a tree. It has two markings on it reminiscent of a snake and what appears to be a snake's skull on the top-most part. This is where Orochimaru returned to after Hiruzen Sarutobi had sealed his hands and he remained here while he experienced the adverse effects of his rotting hands. This is also where Orochimaru kept the hand with his Akatsuki ring which Itachi Uchiha had severed.

In the Land of Rice Fields Arc, Naruto, Sakura, and Jiraiya found this hideout and however was destroyed after the battle with Arashi.

In the Three-Tails Arc, this hideout was found by the Kakashi Hatake-led Team Kurenai after receiving information from an ANBU on its whereabouts. After they located the by then abandoned hideout, a booby-trap was set off and the hideout flooded.

Kusagakure
This hideout is located somewhere in Kusagakure or there about. Another hideout built entirely underground with the entrance being at the foot of a rock formation. Within the hideout itself are numerous rooms and a statue of a gigantic snake coiled up with candles in its eyes. This is where Sai attempted to carry out his secret mission to kill Sasuke Uchiha. This is also where Sakura and Naruto reunited with Sasuke after two years. The confrontation culminated with a portion of the hideout being destroyed.

Southern Hideout
The is located on an island in the middle of an ocean somewhere possibly off the coast of the Land of Waves. It was filled with prison cells and wardens were also employed to keep the prisoners in check, one such person was Karin who later join Sasuke's team Hebi.

Northern Hideout
The is one of Orochimaru's grander hideouts in terms of size. It is one of the hideouts that Orochimaru used to conduct his human experiments with the cursed seal. Jūgo was kept here since he was the origin of the cursed seal. After Orochimaru's demise, Sasuke freed Jūgo from here and had him join his team.

Eastern Hideout
Very little is known about the but it was also built underground. This was where Orochimaru in one last failed attempt, tried to take over Sasuke Uchiha's body. Sasuke, however, was the one to take over Orochimaru's body instead. It was also filled with large glass cylinders with liquids in it. Here was where Suigetsu Hōzuki was kept and experimented on until Sasuke Uchiha freed him from confinement.

Unknown Hideout
After Suigetsu and Jūgo escaped the samurai prison of the Land of Iron, they travelled to a hideout in an unknown location. It has the mouth of a snake for an entrance and several snake-like candle-holders on the walls. Here, Suigetsu found a secret chamber after Jūgo had attacked him and broken a false wall. Within his secret chamber, Suigetsu claims to have found invaluable information that may turn the tides of war in their favour.

Island Laboratory
This laboratory is built into the side of a small island located in a large body of water. It is first seen when Orochimaru brings Kabuto to the building to explain the reason he was attacked by Nonō and how it was part of a plan engineered by Danzō. Orochimaru then explains to Kabuto that the building they are in is a medical laboratory created for him to use.

The lab is covered in medical equipment, ranging from scalpel to test-tubes. The lab's most unique feature, however, is the large blood extracting machine in the center of the room. One of the room's massive walls is covered in ceiling length shelves stacked full of jared animals, books, scrolls, crates, and tools. Kabuto later comes here to transfuse blood of Orochimaru's true form into himself. In the background, several tanks contain large snake-like entities. One of them had an upper body that looked human. Several years later, the same tanks are noticeably empty.

Demon Island Laboratory
This laboratory, appearing in the anime-only Land of the Sea Arc, is located in the Demon Island of the Land of the Sea. It is an experimental laboratory with many transgenic creatures contained in test pillars. Some time before the start of the series, Amachi, the lead scientist, and Orochimaru terrorised the people of the Land of the Sea by kidnapping the citizens for their deadly experiments, which included making the ultimate underwater ninja, an idea later abandoned by Orochimaru.