Scaling the tallest mountain on earth would take you not to Mt. Everest, but to Mauna Kea on the Big Island of Hawaii. Although this peak reaches only 13,803 feet above sea level, the mountain extends another 19,700 feet below the surface, for a total of 33,500 feet from base to summit. At 29,035 feet, Mt. Everest reaches the highest elevation on Earth, but measured from sea level to summit, it’s nearly a mile shorter than the partially submerged Mauna Kea.

Underwater landforms

Beneath the smooth ocean surface extends an underwater landscape as complex as anything you might find on land. While the ocean has an average depth of 2.3 miles, the shape and depth of the seafloor is complex. Some features, like canyons and seamounts, might look familiar, while others, such as hydrothermal vents and methane seeps, are unique to the deep.

Continental shelf

Starting from land, a trip across an ocean basin along the seafloor would begin with crossing the continental shelf. The continental shelf is an area of relatively shallow water, usually less than a few hundred feet deep, that surrounds land. It is narrow or nearly nonexistent in some places; in others, it extends for hundreds of miles. The waters along the continental shelf are usually productive, both from light and nutrients from upwelling and runoff.

Abyssal plains

Continuing your journey across the ocean basin, you would descend the steep continental slope to the abyssal plain. At depths of over 10,000 feet and covering 70% of the ocean floor, abyssal plains are the largest habitat on earth. Sunlight does not penetrate to the sea floor, making these deep, dark ecosystems less productive than those along the continental shelf. But despite their name, these “plains” are not uniformly flat. They are interrupted by features like hills, valleys, and seamounts (underwater mountains that are also hotspots for biodiversity).

Mid-ocean ridge

Rising up from the abyssal plain, you would encounter the mid-ocean ridge, an underwater mountain range, over 40,000 miles long, rising to an average depth of 8,000 feet. Tracing their way around the global ocean, this system of underwater volcanoes forms the longest mountain range on Earth.

Ocean trenches

After scaling the mid-ocean ridge and traversing hundreds to thousands of miles of abyssal plains, you might encounter an ocean trench. The Mariana Trench, for example, is the deepest place in the ocean at 36,201 feet.

Finally, you would ascend tens of thousands of feet back up the continental slope and across the continental shelf. Your journey across an ocean basin would end on the shore of another continent.

