To someone who can't swim, the"deep sea" may be just above ankle level in the ocean. So what do scientists mean when they refer to the"deep sea"?

As shown in the illustration below, the ocean floor is divided into three major areas: the continental shelf, the con- tinental slope, and the deep ocean basin.

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 globe.

The Mid-Ocean Ridge marks one of the most geologically active areas on Earth. It is where the planetÕs crustal plates are moving apart. It is where new seafloor is being born, giving rise to hydrothermal vents and volcanoes.