What is the name of the optional boss in Final Fantasy VII found underwater?

In Final Fantasy VII, the optional underwater boss is Emerald Weapon.

Emerald Weapon is one of the Planet’s Weapons, colossal beings awakened to defend Gaia against threats such as Jenova and Sephiroth’s Meteor. Unlike Diamond Weapon, whose encounter is part of the main story, Emerald Weapon is an optional superboss encountered beneath the ocean in the original Final Fantasy VII.

The player must use the submarine to locate and collide with Emerald Weapon in the underwater world map. The battle is infamous for its enormous 1,000,000 HP, powerful attacks, and the 20-minute time limit normally imposed by the submarine’s oxygen gauge. Equipping the Underwater materia removes that timer, making the fight more manageable but not easy.

Emerald Weapon is often mixed up with Ruby Weapon, another optional superboss found in the desert near the Gold Saucer. Ultimate Weapon is fought during the story and later becomes a roaming optional encounter, while Diamond Weapon is a mandatory story boss in the western releases. Emerald and Ruby are generally regarded as the game’s ultimate post-game challenges.