Oceans of the earth - What lies under them?

Nearly three-quarters of the earth's surface is covered by water.
The land under the oceans is made up of mountains, valleys and large, flat plains, just like the land on the continents.
It even has volcanoes!

Where did the oceans come from? 

Some scientists believe that the oceans were formed about 4 billion years ago.
At this time, hot rocks inside the earth cooled down and water vapor in the air fell as rain.
The rain gradually filled the low places in the earth's crust and formed the first oceans.

How do we know what lies at the bottom of the ocean? 

Scientists who study the ocean are called oceanographers.
With the help of very accurate instruments, they have made maps of the ocean floor.
Modern oceanographers use a device called an echo sounder to measure the depth of an ocean.
An echo sounder sends down sound waves to the ocean floor, and measures how long it takes for them to echo back to the surface of the water.


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