Imagine that you are at the beach. There is sand between your toes, and
you can feel the Sunís warmth on your face. You look out at the ocean.
You can see waves, gradually rising and breaking on the beach. Maybe
there is a ship visible somewhere offshore.
Now think about what you cannot see at the beach, the sea floor. If
you were to swim out into the ocean, where the water is clear, you might
be able to see the bottom. Maybe you would see a coral reef or other sea
life. However, if you were to swim further offshore, eventually you
would not be able to see the bottom. As the ocean gets deeper, seawater
absorbs more and more light until nothing is visible. The sea floor
becomes a dark area, which you cannot see or observe.