Inside Jabu-Jabu's Belly

Inside Jabu-Jabu's Belly is the third dungeon in Ocarina of Time and is set inside Lord Jabu-Jabu's body. To enter, Link must drop a fish from his bottle in front of Jabu-Jabu, causing him to open his mouth and suck both the fish and Link inside. While inside, Link must fight through a variety of parasitic enemies, including an especially large tentacle-like organism, with electrified appendages in the shape of unstable pillars, and jellyfish-like creatures called Bari, which become muc easier to kill after finding the Boomerang, the weapon found inside the dungeon. Shortly thereafter, Link locates Ruto, who commands him to help her recover the final Spiritual Stone, the Zora's Sapphire. You must carry Ruto throughout many rooms to help her find it. You enter a large room, with the Zora Sapphire in the middle.

After taking the Zora's Sapphire, Ruto is captured and Link is ambushed by a Big Octo, which must be defeated in order to continue. After defeating it, Link continues until he eventually locates the creature causing Jabu-Jabu's discomfort: a parasite called Barinade, the dungeon's boss. Link eventually defeats Barinade, and rescues Princess Ruto and along with her the Zora's Sapphire.

Note
In the room where you fight Big Octo, if you attempt to enter the room without Ruto, you cannot get to the Zora Sapphire on your own.