A sea is a large body of salt water that is surrounded in whole or in part by land.[a] More broadly, "the sea" is the interconnected system of Earth's salty, oceanic watersвЂ”considered as one global ocean or as several principal oceanic divisions. The sea moderates Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been traveled and explored since prehistory, the modern scientific study of the seaвЂ”oceanographyвЂ”dates broadly to the British Challenger expedition of the 1870s. The sea is conventionally divided into up to five large oceanic sectionsвЂ”including the International Hydrographic Organization's four named oceans (the Atlantic, Pacific, Indian, and Arctic) and the Southern Ocean; smaller, second-order sections, such as the Mediterranean, are known as seas.
The sea is an essential aspect of human trade, travel, mineral extraction, and power generation. This has also made it essential to warfare and left major cities exposed to earthquakes and volcanoes from nearby faults; powerful tsunami waves; and hurricanes, typhoons, and cyclones produced in the tropics. This importance and duality has affected human culture, from early sea gods to the epic poetry of Homer to the changes induced by the Columbian Exchange, from burial at sea to Basho's haikus to hyperrealist marine art, and inspiring music ranging from the shanties in The Complaynt of Scotland to Rimsky-Korsakov's "The Sea and Sinbad's Ship" to A-mei's "Listen to the Sea". It is the scene of leisure activities including swimming, diving, surfing, and sailing. However, population growth, industrialization, and intensive farming have all contributed to present-day marine pollution. Atmospheric carbon dioxide is being absorbed in increasing amounts, lowering its pH in a process known as ocean acidification. The shared nature of the sea has made overfishing an increasing problem.
Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. At any given place, the water rises over the course of the tidal cycle to a maximum height known as "high tide" before ebbing away again to a minimum "low tide" level. As the water recedes, it uncovers more and more of the foreshore or intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude. Tidal bores can occur at the mouths of rivers, where the force of the incoming tide pushes waves of seawater upstream against the current. At Hangzhou in China, the bore can reach 9 meters (30 ft) high and travel up to 40 km (25 mi) per hour.
There is a broader spectrum of higher animal taxa in the sea than on land, many marine species have yet to be discovered, and the number known to science is expanding annually. Some vertebrates such as seabirds, seals, and sea turtles return to the land to breed but fish, cetaceans, and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats. One of these is the surface film whichвЂ”despite being tossed about by the movement of wavesвЂ”provides a rich environment and is home to bacteria, fungi, microalgae, protozoa, fish eggs, and various larvae
Since the development of coordinated fleets of ships capable of landing an invasion force, naval warfare has been an important aspect in the defense (or conquest) of maritime states. The first naval battle in recorded history saw Suppiluliuma II of the Hittites burn a Cypriot fleet at sea in 1210 BC. Shortly after, the fleets of the Sea Peoples disrupted the entire Eastern Mediterranean: over a period of about 50 years, raids and invasions violently destroyed nearly every coastal city between Pylos and Gaza. As empires grew and their armies became too large to live off the lands through which they passed, disruption of their supply fleets also became a powerful tactic. The 480 BC Battle of Salamis largely determined the course of the Persian Wars not because of its inherent damage (however considerable) but because Themistocles's deception and superior strategy left the Athenians capable of disrupting sea-borne supplies at will and potentially striking at the pontoon bridges across the Hellespont, cutting off the Persians' line of retreat. During the age of wooden ships, however, great fleets were burdensome to maintain and always liable to destruction by contrary weather, most famously in the case of the two kamikaze typhoons that destroyed the Mongol invasions of Japan in AD 1274 and 1281.