The sea is very big. When you look at the map of the world, you will find there is more water than land. The sea covers three quarters of the world. It looks very pretty when the sun is shining on it. It can be very rough when there is a big wind. The sea is also very deep in some places. Some parts of the sea are very shallow. But in some places the depth of the sea is very great. There is one spot, near Japan, where the sea is nearly 11 kilometers deep! The highest mountain in the world is about 9 kilometers high. If that mountain were put into the sea at that place, there would be 2 kilometers of water above it! The sea is salty. Rivers, which flow into the sea, carry salt from the land into the sea. Some parts of the sea are more salty than other parts. There is one sea, called the Dead Sea, which is very salty. It is so salty that swimmers can't sink! Fish can't live in the Dead Sea! The sea can be very cold. Divers, who go deep down in the sea, know this. On the top the water may be warm. When the diver goes downwards, the sea becomes colder and colder. Another thing happens. When the diver goes deeper, the water above presses down on him. It squeezes him. Then the diver has to wear clothes made of metal. But he cannot go very deep. Some people who wanted to go very deep used a very strong diving ship! They went down to a depth of eleven kilometers. Why can't people go very deep into the sea?
Because of the coldness of the water. Because of the living things in the sea. Because of the pressure of the water. Because of the salt in the sea.