The book by V.V. Fedorov talks about underwater research in the Atlantic, Indian and Pacific oceans, which were carried out using domestic manned vehicles “Tinro-2”, “Sever-2” and “Omar”. The author personally took part in more than a hundred dives, making visual observations at depths of up to 1500 m. The reader learns about what strange fish, crabs, mollusks, coelenterates, sponges and other animals live in the depths of the seas and oceans. Some of these animals have been photographed during dives and can be seen in their natural habitat. The dramatic situations in which Russian hydronauts sometimes found themselves, who always found a way out of difficult situations, are described. As a result of underwater research, a number of discoveries of marine and ocean fishing areas were made, which were transferred to the development of the Russian fishing industry. The book is illustrated with unique photographs taken during underwater dives. There are also photographs of manned underwater vehicles and working moments of underwater research. The book is intended for a wide range of readers, including middle and older schoolchildren, students of geographical, geological and biological faculties of universities and pedagogical universities, teachers of schools, lyceums and gymnasiums, as well as all those who is interested in the nature of the ocean.