The Nartec are a ocean-dwelling race of amphibious fish-men native to Earth that aren't actually aliens, but a mutated human subspecies. The Nartec appeared in Book #36: The Mutation and were possibly inspired by the legend of Atlantis. They presumably died off sometime during or after the war, and were never revisited during the series.
They live in a giant cave under the ocean off the coast of the Animorphs' home town. According to the ruler of the Nartec, Queen Soco, the amphibian race once lived on 'the Surface', on an island in the 'Great Ocean.' Over decades, the sea began to swallow up the island. Over generations, the islanders built a towering wall around the island, which grew and grew. Eventually, they were completely submerged, but enclosed by the wall, whose perimeter bent over, touching each other to create a dome. However, according to Ax, this story is highly improbable. What is certain, however, is that the submerged humans adapted to their new environment. Queen Soco referred to this as a "supreme act of will". But soon it became apparent that the 'Narna Rocks', a highly radioactive material that serves as the Nartec's source of light, was the cause of the rapid mutation of the Nartec to their present form, in the space of a few centuries.
Despite being apparently humanoid, the Nartec have huge orange eyes that consume much of their faces. Their skin is blue and slippery, and their hands and feet are webbed in order to improve hydrodynamic performance. Finally, the Nartec possess a dual respiratory system. While retaining their human lungs, which they use on land, they have gills on their necks in order to filter oxygen from the water. Jake and Cassie both draw disturbing parallels to the amphibian Hork-Bajir Visser Three's medical staff experimented on. The Nartec move fluidly in the water, but are weak and slow on land, with a few exceptions.
The Nartec culture appears deplorably degenerate to the visiting Animorphs. It seems backward and extremely inhumane to them. The Nartec have stolen human seagoing vessels which have sunk over the millennia. The Nartec 'Searchers' are deployed to retrieve wreckage, as well as any surviving humans, whom the amphibians refer to as 'Surface-Dwellers'. Ship parts, flotsam and jetsam are used to build furniture, houses, and other items. Sunken ships, some as large as aircraft carriers, that have remained intact are placed in giant galleries on the outskirts of the only Nartec city the Animorphs have seen. The Animorphs' views of the Nartec 'wonders' vary on two different extremes: the way the city was constructed from ship parts, and the gallery, is incredible; and yet, the Nartec technology was merely stolen from humans. The Nartec militia is equipped with a myriad of collected human arms, from bows and arrows to sub machine guns.
It appears as though no accolade can be awarded the Nartec for cultural or scientific achievement. The Nartec are a race of scavengers, using stolen human and Yeerk technologies. However, they have developed a technique of extracting human DNA for reproduction.
Society among the Nartec is comparable to feudalism, which appears to have changed little over centuries. The royal class and aristocracy live in the centre of the town. The bourgeoisie live further out, while the traders, artisans and manufacturers live closer to the outskirts. Finally, criminals and the poor make out an existence outside the town. Cassie makes a remark about the sheer backwardness of Nartec discrimination.
Human prisoners retrieved from sunken vessels have appeared to have been treated well as guests, at first, but are then subjected to horrible torture. Nartec 'doctors' inject a solution of 'ablata' weed, which 'renders subjects peaceful and compliant'. This drug merely sedates the victims; it does not put them into sleep, or mask the pain. After dissecting their victims alive, the Nartec remove all organs and stuff them with an unidentified material, and are then put on displays on the human vessels as 'educational exhibits', which the Animorphs found shocking, disturbing, and disgusting.
The Nartec are a dying race. The radioactive rocks in their underwater world ensured their survival, but also sealed their demise. Most mutations are detrimental. The population and life span of the Nartec have dropped over an unspecified amount of time. Infant mortality has been rising as a result of birth defects. The dropping population has forced the amphibian race to inbreed, causing dangerously low fertility. The doomed Nartec have resorted to desperate measures to survive. The main purpose of dissecting the humans is to extract DNA to be somehow transplanted into Nartec females. It has proven a risky process, as a large number of newborns have not inherited the unique Nartec traits needed for survival. Queen Soco's final solution almost came to fruition by chance. By capturing Visser Three's ocean-venturing vessel, the Sea Blade, the Nartec planned to use a powerful, advanced piece of technology to conquer the 'Cultures of the Sun'. The Animorphs came to the conclusion that the Nartec would fail; however, they still possessed nuclear weapons from Soviet and American submarines, although the Animorphs weren't sure if the Nartec were able to breach the PAL arming codes.