For millennia man has exploited and often destroyed the riches of the land. Now man covets the wealth of the oceans, which cover nearly three-quarters of the earth, but the scramble for minerals and oil, for new underwater empires, could heighten international tensions and set a new and wider stage for world conflict. Even the most conservative estimates of resources in the seabed stagger the imagination. In the millions of miles of ocean that touch a hundred nations live four out of five living things on earth. In the seabed, minerals and oil have been proved to exist in lavish supply. The oceans are a source of pure water and food protein of drugs and building materials they are even possibly a habitat for man himself and a key to survival for the doubling population on the land. A few years ago, practical men dismissed speculations about wealth in the sea. That is economic foolishness, they said. It will never be economically profitable to exploit the seabed, no matter how great the riches to be found there. Unfortunately, they underestimated the lure of gold as the mother of invention. Yet the pessimists may be proved right. In these pioneer years of the Ocean Age, the damage done sometimes seems to exceed the benefit gained. Beaches from England to Puerto Rico to California have been soaked in oily slime. Fish and wildlife have been destroyed. Insecticides, seeping into the rivers and then the oceans, have killed fish and waterfowl and revived fears that other lethal chemicals may contaminate our waters when they are used as garbage dumps. The future disposal of increasing amounts of atomic waste is an unresolved problem. Millions of acres of offshore seabed have been leased for drilling. Largely in ignorance, we are tinkering with our greatest source of life. The incredible magnitude of the ocean's resources can be measured by just one isolated example: the metal content of manganese nodules. These lumps of mineral on the ocean floor were once regarded as a curiosity with no economic value. One study of reserves in the Pacific Ocean alone came up with an estimate that the nodules contained 358 billion tons of manganese, equivalent, at present rates of consumption, to reserves for 400,000 years. The nodules contain equally staggering amounts of aluminum, nickel, cobalt, and other medals. Most of these resources exist at great depths, from 5,000 to more than 15,000 feet below sea level. Yet within five to ten years the technology will exist for commercial mining operations. This will make available virtually unlimited metal reserves. More familiar to most of us is the accelerated pace of offshore drilling that now extends more than 50 miles out to sea and accounts for 15 percent of U.S. oil production. In the twelve years between 1955 and 1967, offshore production of crude oil increased from seven million to 222 million barrels. Estimates of known reserves of natural gas have more than tripled in the past 15 years, and each advance of scientific exploration of the ocean beds brings to light new finds that would gladden the eye of the most hardened veteran of the California gold rush. Perhaps the least developed resource, and one of critical importance to spiraling population figures, is the use of the seas for farming techniques or aquaculture. Present method of fishing can only be compared with primitive hunting with a bow and arrow if fish were cultivated like livestock, the present world fish catch could easily be multiplied five — or as much as ten fold. The production of protein concentrate and the distillation of fresh water are still experimental in an economic sense there is no reason to believe that they too cannot become both useful and profitable. Aquaculture could also be applied to a variety of marine plant life. The principal subject discussed in this passage is ______.