A sea is a large body of water that is part of surrounding waters or is surrounded in whole or in part by land. More broadly, the sea (with the definite article, also called the World Ocean) is the interconnected system of the Earth's salty, oceanic waters, including the IHO's four named oceans - the Atlantic, Pacific, Indian, and Arctic - and the waters of the Southern Ocean, either considered separately or included within the other four. The sea moderates the Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been travelled and explored since prehistory, the modern scientific study of the sea- oceanography - dates broadly to the British Challenger expedition of the 1870s. The sea is conventionally divided into four or five large sections, such as the Pacific, called oceans while smaller sections, such as the Mediterranean, are known as seas.

Owing to the present state of continental drift, the Northern Hemisphere is now fairly equally divided between land and sea (a ratio of about 2:3) but the South is overwhelmingly oceanic (1:4.7). Salinity in the open ocean is generally in a narrow band around 3.5% by mass, although this can vary in more landlocked waters, near the mouths of large rivers, or at great depths. About 85% of the solids in the open sea are sodium chloride. Deep-sea currents are produced by differences in salinity and temperature. Surface currents are formed by the friction of waves produced by the wind and by tides, the changes in local sea level produced by the gravity of the Moon and Sun. The direction of all of these is governed by surface and submarine land masses and by the rotation of the Earth (the Coriolis effect).

Former changes in the sea levels have left continental shelves, shallow areas in the sea close to land. These nutrient-rich waters teem with life, which provide humans with substantial supplies of food - mainly fish, but also shellfish, mammals, and seaweed - which are both harvested in the wild and farmed. The most diverse areas surround great tropical coral reefs. Whaling in the deep sea was once common but whales' dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting. Oceanography has established that not all life is restricted to the sunlit surface waters: even under enormous depths and pressures, nutrients streaming from hydrothermal vents support their own unique ecosystem. Life may have started there and aquatic microbial mats are generally credited with the oxygenation of Earth's atmosphere; both plants and animals first evolved in the sea.

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 Viking funerals 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.


Humans have travelled the sea since prehistoric times, originally on rafts and in dugout, reed, and bark canoes. Most of the early human migrations occurred over land: even areas now separated by open sea such as the Americas, Japan, and Britain were accessible by land bridges or fast ice during the last ice age. However, the dwarf Flores man probably needed to cross a 19-kilometer (12 mi) wide strait from Sundaland to reach Komodo and, although the exact details remain uncertain, the ancestors of Australia's Aborigines must have crossed the broader deep-sea Wallace Line to Near Oceania tens of thousands of years ago.[g] Despite earlier theories, modern bathymetric soundings now suggest that even the earliest settlement of the Philippines required crossing deep water at the Mindoro Strait or the Sibutu Passage.

The hunter-gatherer Ortoiroid people began spreading through the Caribbean from Venezuela's Orinoco valley by at least the 6th millennium BC. Around the same time, Mesopotamians were using bitumen to caulk their reed boats and, a little later, masted sails.Lothal in India boasted the earliest known dock around 2400 BC. By c. 2000 BC, Austronesians on Taiwan had begun spreading into maritime Southeast Asia.

From 1300 to 900 BC, the Austronesian "Lapita" peoples displayed great feats of navigation, reaching out from the Bismarck Archipelago to as far away as Fiji, Tonga, and Samoa. Their descendants continued to travel thousands of miles between tiny islands on outrigger canoes: the Austronesians of the Sunda Islands settled Madagascar off southeast Africa before AD 500 and the Polynesians settled the Hawaiis before 800, Easter Island before 1200, and New Zealand shortly after.

The Egyptian pharaoh Necho II initiated construction on a canal which eventually linked the Mediterranean and Red Seas around 600 BC. Herodotus records Egyptian claims that he also commissioned a 3-year-long expedition which circumnavigated Africa from the Red Sea to the Nile delta.

Around 500 BC, the Carthaginian navigator Hanno left a detailed periplus of an Atlantic journey that reached at least Senegal and possibly Mount Cameroon; and the Greek Pytheas left another exploring the seas around Great Britain around 325 BC. The massive 3rd-century BC Lighthouse of Alexandria was considered one of the Seven Wonders of the World.

In the 2nd century, the Alexandrian Ptolemy mapped the known world, using the "Fortunate Isles" as his prime meridian and including details as distant as the Gulf of Thailand. A modified form was used by Columbus for his voyages.

In the Mediaeval era, the Vikings used clinker-built ships to colonize Iceland, Greenland, Canada, and Russia. A compass showing magnetic north is first attested - in the form of a "south-pointing spoon" - in the 1st-century Chinese Lunheng.

The first evidence of its use in Chinese maritime navigation, however, dates to Zhu Yu's c. 1115 Pingzhou Table Talks. Alexander of Neckham's De naturis rerum, the first European mention of a magnetized needle, dates to 1190 and immediately notes its use among sailors. Latitude (the ship's position ranging from 0 at the equator to 90° north and south) could be determined by inclinometers - including the astrolabe, sextant, and Jacob's staff - measuring the angle between the horizon and heavenly bodies like the sun or moon. Accurately determining longitude (the ship's position east or west of some fixed point) proved much harder. Read more ...

In the News ...

  Massive Sinkholes Break Open As Dead Sea Shrivels   ABC - March 20, 2015
There are more than 3,000 sinkholes on the banks of the Dead Sea -- and they're multiplying exponentially, according to environmentalists, as the body of water dries up. "It's nature's revenge," said Gidon Bromberg, the Israeli Director at EcoPeace Middle East, an organization that brings together Jordanian, Palestinian and Israeli environmentalists to protect their shared environmental heritage. These sinkholes are a direct result of the inappropriate mismanagement of water resources in the region. More than 1,400 feet below sea level, the Dead Sea is the lowest point on land. The first sinkhole was spotted in the 1980s. By 1990, there were 40, and 15 years later new chasms are breaking open every day.

Scientists drill beneath Dead Sea in search of priceless data   PhysOrg - December 22, 2010
Rock samples that have been underwater for millions of years are likely to be better preserved, they say, than samples taken from under an exposed surface, which can be damaged by aridity and erosion. As a result, the Dead Sea bore hole is expected to contain priceless information about the planet's past and to offer insight on its future. Expectations are high that the lowest place on Earth can answer questions on climate change, earthquake risk and untapped natural resources.

Mediterranean most threatened sea on Earth   PhysOrg - August 2, 2010
The Mediterranean Sea's exquisitely rich mix of flora and fauna is more threatened than marine life anywhere else on Earth, according to a landmark scientific survey released Monday.

Mediterranean Sea Getting Saltier, Hotter   Live Science - May 24, 2010
Western Mediterranean increases by 0.0036 degrees Fahrenheit (0.002 degrees Celsius), and its salt levels increase by 0.001 units of salinity, researchers monitoring the sea found. The change is consistent with the expected effects of global warming. These changes may sound like small beans, but they have been building up at a faster pace since the 1990s, the study, detailed in the April 1 edition of the Journal of Geophysical Research, suggests. The results show a consistent trend, "but to confirm this accelerating trend, we need to monitor it over the years to come," said study author Manuel Vargas-Yanez of the Spanish Institute of Oceanography.

Red Sea coral reefs get their complex shape from an ancient 'seabed template'.   BBC - February 23, 2010
Red Sea coral reefs get their complex shape from an ancient 'seabed template', say scientists. Their distinctive appearance can be seen clearly in satellite images of the region and has its origin in seabed erosion thousands of years ago. The scientists say the corals have simply adopted and accentuated the pattern created in once-exposed rock moulded by heavy rains.

Colossal Flood Created the Mediterranean Sea   Live Science - December 9, 2009
The Mediterranean Sea as we know it today formed about 5.3 million years ago when Atlantic Ocean waters breached the strait of Gibraltar, sending a massive flood into the basin. Geologists have long known that the Mediterranean became isolated from the world's oceans around 5.6 million years ago, evaporating almost completely in the hundreds of thousands of years that followed. Scientists also largely agree that the Mediterranean basin was refilled when the movements of Earth's crustal plates caused the ground around the Gibraltar Strait to subside, allowing the ocean waters of the Atlantic to cut through the rock separating the two basins and refill the sea. But exactly how the waters cut their way through and how long it took them to do so wasn't known.

Ancient Mediterranean flood mystery solved   BBC - December 9, 2009
Research has revealed details of the catastrophic Zanclean flood that refilled the Mediterranean Sea more than five million years ago. The flood occurred when Atlantic waters found their way into the cut-off and desiccated Mediterranean basin. The researchers say that a 200km channel across the Gibraltar strait was carved out by the floodwaters. Their findings, published in Nature, show that the resulting flood could have filled the basin within two years.