The Sea of Eternity is a location on Eternia that features in He-Man and the Masters of the Universe.
The Sea of Eternity lies deep in the heart of the Mystic Mountains. It feeds a river which supplies the Vine Jungle and the Evergreen Forest with water. These two great forces in turn supply all of Eternia's air with oxygen.
The Insect People inhabit underground caves that lead to Sea of Eternity.
At one time Skeletor built a dam to stop the water from the sea flowing towards the river, cutting off the oxygen for the entire planet. Working together, He-Man and Garth of the Insect People managed to defeat Skeletor and restore the order to nature and the planet. Furthermore, the Insect People agree to guard the Sea to ensure that Skeletor does not attempt that scheme again.
Because of the rich oxygen and the many unusual powers within the water, it caused Garth to evolve into a new form, one that would normally take his people centuries to evolve into.