Earth
Earth's global ocean, which covers nearly 70 percent of the planet's surface, has an average depth of about 2.5 miles (4 kilometers) and contains 97 percent of Earth's water. Almost all of Earth's volcanoes are hidden under these oceans. Hawaii's Mauna Kea volcano is taller from base to summit than Mount Everest, but most of it is underwater. Earth's longest mountain range is also underwater, at the bottom of the Arctic and Atlantic oceans. It is four times longer than the Andes, Rockies and Himalayas combined.