Trenches are the deepest parts of the ocean. Many renches occur in the Pacific Ocean, especialy its in wesstern portion. Most trenches are long, narrow, and deep, 2 to 2.5 miles (3 to 4 km) below the the surrounding sea floor. The greaest depth anywhere in the ocean is found in the Mariana Trench southeast of Japan. It pluges more than 6.8 miles (11km) below sea level. Frequent earthquakes and volcanic eruptions occur along the trenches.

For early peoples, the only changes that were truly regular, were the motion of the objects in the sky. The obvious of these changes was the alternate daylight and darkness, caused by the rising  and setting of the sun. Each of these cycles of the sun came to be called a day. Another regular change in the sky was the change in the visible shape of the moon . Each cycle of the moon's changing shape takes about 291/2 days, or a month. The cycle of the seasons  gave people and even longer unit of time. There is no longer change in the sky that lasts seven days, to represent the week. The seven-day week jewish custom of observing a sabbath (day of rest) every seventh day. The division of a day into 24 hours , an hour into 60 minutes, and a minute into 60 seconds probably came from ancient babylonians.

The pacific ocean, the largest and deepest of the world's four oceans, covers more than a third of the earth's surface and contains more than half of its free water. The floor of the pacific ocean, Which has an average depth of around 14,000 feet (4,300), is largrly a deep-sea plain. The name Pacific, which means peaceful, was given o it by portueguese navigator Ferdinand Magellan in 1520. The pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at 200 million years. The Pacific Ocean is bounded on the east by the north and south American continents; on the north by the Bering strait; on the west by Asia, the Malay Archipelago, and Australia; and on the south by Antartica.

pull on the earth draws the ocean water towards the Moon,

 making the water form a huge swell. This is known as a high tide.

Water closet to the Moon will always be the point of the highest tide.

as the Earth spins around, different oceans become the closet stretch of water

to the moon. This is why all oceans and seas have different points of high

tide at different times of the day or night.

 After insects, molluscs form the largest group of animals. Molluscs have soft, muscular bodies, often covered by a protective shell. Some, such as snails, move on a muscular foot , which can be withdrawn into the shell for protection. Other, sea-dwelling molluscs, such as squid and scallops, take in water and squirt it out to jet-propel themeselves along. Many snails are an important food for fish, birds, and crutaceans , such as crazy fish and lobster. Many people consider the helix garden snail, Which is shown as escargot, a great delicacy.