Can you imagine traveling to work in a one-man submarine? Scientists believe that someday one-man submarines will be as numerous as automobiles today. One says," One day soon, men will walk on the ocean floor as they do on the street. " Perhaps during your life time, people will travel, work and live under the sea.    Ships carrying oil and articles of trade will move beneath the ocean. Passenger ships will be built to travel underwater, where storms cannot delay them or make people seasick. Special underwater ships will be designed for mining, fishing, and exploring unknown areas. For military use, there may be submarines that can fly and then dive underwater to approach the battle area without being seen.    First, however, the human problems associated with living in the ocean will need to be studied in detail. Many of these problems are the same as the problems of living in outer space: pressure, lack of oxygen, and weightlessness. Already scientists have begun to study these problems.     In 1963 and 1964 scientists and Navy divers lived and worked underwater for several days in two projects which were called Sealab I and Sealab II. The Sealab II experiment kept the scientists and divers 215 feet below the surface for thirty days, studying marine biology, temperature, pressure, currents, and ocean bottom geology.    Many puzzling questions remain unanswered, however. Man's warm blood makes it difficult for him to live long in the sea without some kind of warmth. Can our blood be changed to fit new water surroundings? What will happen to our muscles if we live in the water very long? Will they become soft away from our usual land existence? Will bones become weak and our blood pressure change? Scientists are searching for answers.    By studying plants and animals deep in the ocean, scientists may find a cure for some of our most serious human diseases. We may learn to use plants to increase our world food supply. Engineers may learn to use the power of the sea by building huge walls to catch the force of the tides. In the passage, the author is talking to us about the problems of   _
pressure, lack of oxygen, and weightlessness in the sea living, working below the ocean underwater submarine walking on the ocean floor