The sea in a general sense refers to the ocean or world ocean, the body of salty water that covers approximately 71% of the Earth's surface. Used in a particular sense the word sea refers to particular seas either as second-order sections of the ocean, such as the Mediterranean Sea, or as certain large, entirely landlocked, saltwater lakes, such as the Caspian Sea. The sea moderates Earth's climate and has important roles in the water, carbon, and nitrogen cycles. Humans harnessing and studying the sea have been recorded since ancient times, and evidenced well into prehistory, while its modern scientific study is called oceanography. The most abundant solid dissolved in seawater is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and mercury, amongst many other elements, some in minute concentrations.