The continental shelf extends underwater from each of
the major land masses. It's the submerged portion of the continents.
The shelf has features similar to those we see on land, including
hills, ridges, and canyons. The shelf varies in size. It may
be virtually non-existent off some land masses; elsewhere, it
may extend underwater a great distance from shore. The shelf's
average distance is about 64 kilometers (40 mi).
It is beyond the continental shelf that the "deep
sea" begins. The shelf ends at a depth of about 200 meters
(660 ft), which is much deeper than the deepest recorded dive
of a scuba diver, at 145 meters (475 ft). The continental shelf
gives way to the steeper continental slope, which descends about
3,700 meters (12,000 ft) to the deep ocean basin.
Here, the ocean floor deepens sharply, and its features
resemble those on land, only on a much larger scale, with great
plains and mountains. In fact, the Earth's longest mountain range
is underwater. Over 56,000 kilometers (35,000 mi) long, this
mountain range, called the Mid-Ocean Ridge system, snakes around
The Mid-Ocean Ridge marks one of the most geologically
active areas on Earth. It is where the planet's crustal plates
are moving apart. It is where new seafloor is being born, giving
rise to hydrothermal vents and volcanoes.