Underwater volcanoes are described as oases of the sea in the documentary "Underwater Volcanoes - Oases of the Sea" by Free Documentary Nature. These volcanoes, located beneath the ocean's surface, provide a unique habitat for a variety of marine life. The volcanic activity creates a nutrient-rich environment that supports diverse ecosystems and attracts a wide range of animals, including colorful fish, deep-sea creatures, and even some species that are found nowhere else on Earth. Scientists are fascinated by these underwater oases and continue to study them to learn more about the impact of volcanic activity on marine biodiversity.

Overall, the documentary highlights the importance of underwater volcanoes as crucial ecosystems in the ocean and showcases the beauty and wonder of these dynamic underwater landscapes.