Almost three-fourths of the earth is under the ocean. Until a few years ago, people did not know what the ocean bottom was like.
The ocean bottom is different from what we thought. After World War I scientists made a new machine. This machine told them what the bottom of the ocean was like. The machine told how deep the ocean is in each place. For a long time many people thought the ocean bottom was flat. Now we know that there are large mountains and deep holes in the ocean bottom.
“... what the ocean bottom was like.” refers to
