Because there are fossils left as evidence of the existence.
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The internment camps were also known as relocation centers, once it was a kind of prison where people were locked for some reason. For example, during the World War II, a lot of Japanese people were relocated in these internment camps, because there was fear that the Japanese people that lived in US, would react to the US attacks in the war.
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By March 1942, the Japanese invasion of the Philippines<span> had compelled </span>MacArthur<span>to withdraw his forces on Luzon to Bataan, while his headquarters and his family moved to Corregidor Island.</span>