The ocean floor

Imagine all the water in the sea is gone.

You would see mountains, hills, flat plains and deep valleys. That is the ocean floor!

If you could drain all the water from the ocean, you would see a landscape as amazing as anything on land. NOAA, the U.S. ocean agency, explains that beneath the smooth surface lie canyons, mountains, plains and deep trenches.

The ocean is deep: NOAA Ocean Exploration gives its average depth as about 3,682 meters, a 2010 estimate made by NOAA and Woods Hole Oceanographic Institution scientists using satellites. Some ocean floor features are found nowhere on land, such as hot springs called hydrothermal vents.

NOAA describes the seafloor as an underwater landscape as complex as anything on land. Some features, such as canyons and seamounts, resemble land forms; others, such as hydrothermal vents and methane seeps, are unique to the deep. The study of the shape of the ocean floor is called bathymetry.

NOAA Ocean Exploration's best estimate of the average ocean depth, 3,682 m (12,080 feet), was calculated in 2010 by NOAA and Woods Hole Oceanographic Institution scientists from satellite measurements. Those data showed the seafloor to be bumpier and more mountainous than previously known, which lowered the average from earlier estimates. NOAA explains that the bathymetry is largely a result of plate tectonics.

Near the beach, the sea is not very deep. This is the continental shelf.

Then the floor slopes down, down, down into the deep sea.

Imagine walking along the sea floor from a beach. First you cross the continental shelf, an area of fairly shallow water around the land, usually less than a few hundred feet deep. In some places the shelf is very narrow; in others it stretches for hundreds of miles. Its waters are usually rich in life, because they get sunlight and nutrients.

At the edge of the shelf, the floor drops down the steep continental slope toward the deep ocean.

NOAA's tour of an ocean basin starts on the continental shelf, relatively shallow water usually less than a few hundred feet deep, narrow or nearly absent in some places and hundreds of miles wide in others. Shelf waters are usually productive, supplied with light and with nutrients from upwelling and runoff. National Geographic notes that the continental shelf, not the coastline, marks a continent's true edge.

Beyond the shelf lies the steep continental slope. NOAA's illustrative profile places the shelf at about 300 feet (about 90 m) and the slope between about 300 and 10,000 feet (about 90 to 3,000 m), while stressing that each feature occurs over a considerable range of depths.

Most of the deep sea floor is wide and fairly flat.

It is very dark and cold down there. No sunlight reaches it.

At the bottom of the slope are the abyssal plains. NOAA says they lie more than 10,000 feet (over 3,000 meters) down and cover 70 percent of the ocean floor, making them the largest habitat on Earth.

Sunlight never reaches these depths, so these dark places produce less food for living things than the sunny shelf does. And despite their name, the plains are not perfectly flat: they are broken up by hills, valleys and underwater mountains called seamounts.

Abyssal plains, at depths of over 10,000 feet and covering 70 percent of the ocean floor, are according to NOAA the largest habitat on Earth. With no sunlight reaching the bottom, these ecosystems are less productive than those of the continental shelf.

The plains are interrupted by abyssal hills, valleys and seamounts, underwater mountains that NOAA calls hotspots for biodiversity. In its illustrative profile, an abyssal hill rises about 3,000 feet above the plain and a seamount about 6,000 feet.

A very long line of mountains runs along the sea floor.

It goes all the way around Earth. Hot melted rock comes up there and makes new sea floor.

Rising from the abyssal plains is the mid-ocean ridge, a chain of underwater volcanoes that winds through the global ocean. NOAA says it is over 40,000 miles (about 64,000 kilometers) long, the longest mountain range on Earth, with its tops at an average depth of 8,200 feet (about 2,500 meters).

The ridge is where tectonic plates move apart. Melted rock called magma rises between them and cools into new sea floor. Hot springs called hydrothermal vents form there too. Their discovery in 1977 showed scientists that whole communities of living things can survive without sunlight.

NOAA describes the mid-ocean ridge as a system of underwater volcanoes over 40,000 miles long, rising to an average depth of 8,200 feet: the longest mountain range on Earth. Where plates diverge, magma flows up between them, forming ridges, underwater volcanoes, hydrothermal vents and new oceanic crust.

The discovery of hydrothermal vent ecosystems in 1977 broadened ideas about life. Sunlight had been thought to power the base of every food web, but vent organisms rely on chemosynthesis, using hydrogen sulfide or other chemicals for energy. NOAA notes that the chemistry of vents is suspected to be close to the conditions in which life began. (USGS's This Dynamic Earth gives a different figure for the ridge: more than 50,000 km.)

The deepest places in the sea are long, narrow trenches.

The deepest of all is in the Pacific Ocean. It is called the Mariana Trench.

Trenches are long, narrow, very deep valleys in the sea floor. They form where one tectonic plate slides down beneath another. The deepest place in the whole ocean is Challenger Deep, at the southern end of the Mariana Trench in the western Pacific. The trench runs for several hundred kilometers southwest of the island of Guam.

NOAA Ocean Exploration measures it at about 10,935 meters (35,876 feet) deep. It is named after the British ship HMS Challenger, whose crew first measured the depths of the trench in 1875.

Trenches form at convergent boundaries, where one plate moves beneath another, a process that also causes earthquakes and builds volcanoes, NOAA explains. Challenger Deep, in the southern Mariana Trench beneath the western Pacific, is the deepest known point: NOAA gives about 10,935 m (35,876 ft). It is named after HMS Challenger, whose crew first sounded the trench in 1875.

Measurements vary. A NOAA education page gives 36,201 feet for the Mariana Trench, and other surveys give other values; NOAA Ocean Exploration notes that a search for ocean depths returns a range of results, because the ocean is vast and hard to measure. The trench is far deeper than any mountain is high above sea level.

How do we know what the sea floor looks like?

Ships send down sounds. The sounds bounce back. That tells us how deep the water is.

Long ago, sailors measured depth by dropping a weighted line until it hit the bottom; these were called soundings. Today, NOAA explains, ships use echo sounders that send pulses of sound to the sea floor and time the echoes. Satellites help too, by spotting tiny bumps in the sea surface above underwater mountains.

There is still a lot to discover. NOAA Ocean Exploration says that as of April 2026, only 28.7 percent of the sea floor had been mapped in high detail.

In 1807, President Thomas Jefferson established the U.S. Coast Survey, NOAA's predecessor, to survey and chart the young nation's ports and waterways. Early soundings used a lead line; today multibeam echo sounders are the main tool. Satellites can map the whole seafloor indirectly from small differences in sea surface height and gravity, but NOAA Ocean Exploration stresses that this gives only a general, low-resolution picture.

As of April 2026, it reports, 28.7 percent of the global seafloor had been mapped with modern high-resolution technology, and explorers have seen less than 0.001 percent of the deep seafloor, roughly the size of Rhode Island. Since more than 90 percent of the ocean is deeper than 200 meters, most of it remains to be explored.