Ocean

Oceans are big wet blue things that cover quite a lot of the earth. They are very salty, extremely polluted and contain many delicious edibles.

History
Oceans formed over the course of millions of years after the shiny new Earth cooled enough for water to settle on its surface and to fill up the shallower bits. As the crust drifted about, continents and supercontinents formed and re-formed; the spaces in between these land masses would become individual seas. Pangaea, the most recently-formed of the supercontinents, split up between 250 and 150 million years ago, dividing the large Tethys ocean into several smaller oceans. The Atlantic Ocean is the youngest of these; as measured by the record of changes in the magnetic field in the rocks below it &mdash; it is thought to date back less than a hundred million years.

In historic times, the oceans posed an insurmountable barrier to most cultures. During the Middle Ages, better shipbuilding techniques finally enabled regular and relatively safe crossings. As a result, five or six small countries on the northeastern Atlantic coast succeeded in carving up most of the world into a few massive "sea-borne" empires, the consequences of which remain readily apparent today.

According to some Young-Earthers, the amount of salt in the oceans suggests that seas formed up to 62 million years ago. The Young-Earthers make their argument based on a starting point of zero salt. False assumption &mdash; the oceans actually once had more salt than they have today. This is due to the fact that a large amount of salt is deposited on the shore when seawater evaporates outside of the ocean &mdash; the first great lowering of salinity occurred in the late Precambrian - 1 billion to 541 million years ago - when huge amounts of salt were deposited in Neoproterozoic evaporite basins. These deposits result in a mineral known as "evaporite". If you like bikes, you'll love the long-term global ocean salinity cycle.

In other evidence for the extreme age of the oceans, not long after they formed, iron-manganese nodules started forming, incredibly slowly: see here for more information.

Life
The greatest variety of life occurs in coral reefs, but the vast open ocean has the most productivity.

There are plenty of fish in the sea - the oceans contain more than (insert really big number here) species. They also teem with plant life and invertebrate animals, and many species of aquatic mammal, including dolphins and whales. Most of the really weird stuff, however, is in the deep sea trenches. The deepest recorded sea trench is the Mariana Trench which is thought to be roughly just under 11,000 m, or about 36,000 ft.

Non-conservation
The oceans have also provided food for seagoing communities (a resource currently being depleted faster than it can be replaced), and are also used as a huge rubbish dump. Both of these may well have a permanent effect on sea life &mdash; it is possible that jellyfish may become the sea's dominant life form after all the fish have been eaten or choke on plastic bags.

Whales are probably the most famous example of overfishing (overmammaling?). The result is this:

* The Atlantic population of gray whales went extinct in the late-17th century. It is not listed as a part of IUCN's red list.

Despite the mess whaling made of the marine mammal population, people still do it (even though commercial whaling is banned by international law). As of 1987, Japan has been whaling for "scientific purposes", which apparently include distributing the meat of what may be the second most intelligent species on Earth to school children for lunch, and (one can safely assume) resurrecting commercial whaling.

Relevance to global warming
Oceans store 90% of the "additional" energy retained by the greenhouse effect. Even years that are not "hottest on record" for surface temperature changes tend to be "hottest on record" for the oceans. . In addition to this, sea level rises and ocean acidification are among the most prominent and damaging long-term consequences of unchecked climate change.

Sea also

 * Shark
 * Bermuda Triangle