Ocean floor and ocean trenches

If you leave the beach heading seaward, you go down a kind of staircase. The first step is the continental shelf, the underwater continuation of the continents, shallow and up to about 200 meters deep. This is where almost all coral reefs and most fisheries are located.

Beyond that comes the continental slope, a steep ramp that drops to 3,000 or 4,000 meters. At its base the abyssal plains begin — nearly flat bottoms covered in sediment, between 3,000 and 6,000 meters deep. On average, the oceans are about 3,700 meters deep.

A little less than 30% of the seafloor has been mapped in detail, according to the international Seabed 2030 project. The rest is known only from satellite measurements, which show the large shapes but not the details.

Ocean trenches form where one tectonic plate dives beneath another. They are long, narrow valleys, much deeper than the surrounding seafloor.

The deepest is the Mariana Trench, in the Pacific Ocean, near Guam. The lowest point, the Challenger Deep, lies about 10,935 meters below sea level. If Everest were placed there, the summit would still be more than 2,000 meters underwater.

Each ocean has its deepest point: in the Atlantic, the Puerto Rico Trench; in the Indian, the Java Trench; in the Southern Ocean, the South Sandwich Trench; and in the Arctic, the Molloy Trench.

Point Nemo is the spot in the ocean farthest from any land. It is in the South Pacific, 2,688 kilometers from the three nearest landmasses: Ducie Island, Motu Nui (near Easter Island), and Maher Island in Antarctica.

The name is a tribute to Captain Nemo from the book Twenty Thousand Leagues Under the Sea by Jules Verne. Because it is so remote, the area is used by space agencies for controlled re‑entries of satellites and spacecraft at the end of their service life.

Astronauts on the International Space Station, about 400 kilometers up, are often closer to Point Nemo than anyone on land.

In Fernando de Noronha, dives descend along the walls of the submarine mountain that forms the archipelago, with turtles, sharks and dolphins, and the water often has more than 30 meters of visibility. For experienced divers, the corvette Ipiranga, wrecked near the island, lies at about 60 meters depth.

In Iceland, at Þingvellir National Park, you can snorkel or dive in the Silfra fissure between the North American and Eurasian plates, in water so clear you can see more than 100 meters. A drysuit is required, because the water stays near 2 °C year-round.

Motu Nui, one of the three lands closest to Point Nemo, is a small islet beside Easter Island in Chile, famous for the moai. It is more than 2,600 kilometers from Point Nemo but is the closest a traveler can get to it by land.