On this day in 1959, Mysteries of the Deep dove in theaters, taking audiences under the sea to learn about the world that exists fathoms below. This documentary featurette covers a wide range of sea creatures, big and small, from conception and birth, to their development into adults. Dolphins, octopi, and sea snails are just a few of the sea inhabitants featured, along with more rare and unique fish and other ocean dwellers. Roy E. Disney was heavily involved in the production and wrote the script as well. Many years after the film’s release, Roy stated that of all the nature films he worked on, Mysteries of the Deep was the most important to him, and he remained disappointed that it did not win the Academy Award®, although it was nominated in the category of Best Short Subject (Live Action).