From Halopedia, the Halo wiki
The Pacific Ocean is an ocean on the human homeworld of Earth. The nation of Australia sits between the Pacific and Indian Oceans, while the island nation of Japan is off the mainland coast of Asia in the Pacific. Aranuka was near the center of the ocean, a long distance from other places.
During World War II, the United States Pacific Fleet was crippled by Isoroku Yamamoto at Pearl Harbor.
Sometime prior to 2558, the Pacific Ocean had risen and wiped out the original Aranuka atoll.