Captain Planet and the Planeteers - Season 1 Episode 12 -

Gi is knocked unconscious while investigating ocean pollution. She is rescued by an inhabitant of a secret underwater city called Oceanus. Since Sly Sludge's illegal toxic dumping operation is polluting the surrounding waters, both the sea life and the underwater city are in danger of being destroyed. The Planeteers must race against time to find Gi, and use teamwork to save the people of this underwater paradise.