Sub-Atlantica is an undersea kingdom in "the deepest part of the Atlantic," populated by telepathic, human-sized mermen. Its origins are unknown, and in spite of—or perhaps because of—its very high technology, the humans seem unaware of its existence outside of the traditional Atlantis myth.
Its ambitious king, Nergill, seeks to conquer the surface world, but all of his technology is powered by geothermal vents, and leaving them would render his weapons useless. However, when a partnership with the Decepticons gives him energon-cube technology, his goals are at last within reach.
Sunbow Generation One cartoon
When Laserbeak and Buzzsaw detected an underwater city giving off massive energy readings, Megatron immediately investigated. What he found was Sub-Atlantica: a single, compact city of mermen wielding weapons comparable to the Decepticons'. Shots were traded, but both sides realized they were a match for each other. As Megatron and the Sub-Atlantican King Nergill bantered, they realized they both had something to offer. The energy that Megatron coveted came from geothermal vents, which confined Sub-Atlantica to the ocean floor. But with the portability of energon cubes, Nergill could take his weapons anywhere—even the surface world he yearned to conquer.
So they struck a partnership, and Megatron found that Nergill's ambitions had come with ample preparation. The compactness of the city allowed it to be moved to the surface via "hydro thrusters," and its "dome of invulnerability" force field provided protection. The Sub-Atlanticans themselves were perfectly mobile above water; they just needed water-filled helmets. And their aforementioned hand-held blasters were on par with Transformer weaponry.
But Starscream was openly wary of Nergill, especially since the boastful man-fish communicated telepathically with his subjects. However, Soundwave was able to record the mental communiques and decipher them. While they did reveal that Nergill intended to betray the Decepticons, that didn't stop the 'Cons from working with him. After all, they had the same intent towards him.
Sub-Atlantica surfaced near its first target: Washington, D.C. The Decepticons and Sub-Atlanticans stormed the city and claimed it by the dreadful means of putting a force field around it, then standing around inside. But the Autobots had made it in before the force field was up, and a battle ensued, one which the Autobots lost when Nergill shot them with a paralyzing "magnetic dysfunction ray."
Only the intervention of the Dinobots turned the (ahem) tide. They were able to smash through the force field literally like it was glass. Grimlock eventually got his hands on Nergill's blaster, which he crushed. That somehow freed all the immobilized Autobots, and Megatron and Nergill retreated to Sub-Atlantica.
After repairing some of the damage to DC, the Autobots pursued them. Sub-Atlantica had moved out into the ocean but still hadn't submerged, so the Dinobots started smashing buildings, and Nergill quailed. With inscrutable merman logic, Nergill declared, "Before I let those monsters destroy my city, I'll use the energy stockpiles and blow Sub-Atlantica and everyone on it to nothingness!" Spike overheard, and for some reason, he, Bumblebee, and Wheeljack tried to stop the king. Luckily, they failed.
But even as Sub-Atlantica was self-destructing, Wheeljack carried the apparently unconscious Nergill out. Then, as the Autobots watched the exploding city sink from afar, Spike noted that Nergill had disappeared. His fate remains unknown, as does the answer to the pivotal question: The hell? Mermen?!