Based on a children's picture book of the same name.

On December 5, 1965, a U.S. aircraft carrier is en route from Vietnam to Yokosuka, Japan. While conducting training exercises 80 miles off Okinawa, an A4 strike aircraft is loaded with a B43 hydrogen bomb, but it falls overboard and sinks in 16,000 feet of water. Based on the true story of the USS Ticonderoga, which reached Japan two days later (on the anniversary of Pearl Harbor; nice touch). Neither side revealed the incident, nor that she was carrying atomic weapons in contravention of U.S. treaties with Japan. The plane, its pilot, and the bomb were never recovered, and the incident was only declassified in 1989, causing an outcry in Japan and resulting in this environmentally themed anime. The extra "n" in Ticonderonga may be a genuine error in transcription or an attempt to distance the story from real events, since the anime continues with Ashika, a boy from a Japanese fishing town, contacted by telepathic whales who bring him visions of fearful seamonsters. Then again, if you were a whale given a choice between nuclear contamination and talking to Japanese fishermen, which would you choose?

(Source: The Anime Encyclopedia)