Ever since people took their first hesitant voyage in a boat, they have tried to understand the ocean. Superstitious minds attempted to explain the disasters that occurred at sea, so they reasoned that the gods sent the storms that tormented them, sea monsters snatched ships into watery depths and unsuspecting fleets sailed off the edge of the Earth.

Although they made these explanations quite fanciful, they mistakenly (and unimaginatively) pictured the unseen ocean floor as a flat, sediment-covered plain.

Until the 20th century, our limited knowledge of the ocean floor came from the ancient method of cast­ing a weighted rope overboard to measure the depth of the sea. From the globs of sediment that clung to the weights, we guessed that an oozy mud covered the whole of the ocean floor, obscuring sea bottom, sunken ships, treasure and even lost civilizations.

Beginning in the 1920s, with the aid of the newly developed echo sounder, startled oceanographers began to realize that the ocean floor was extremely irregular and not the flat smooth plane it was thought to be.

To chart the ocean depths, the echo sounder meas­ures the time it takes sound pulses to travel from a ship on the surface to the ocean floor, and return as echoes. Echoes that bounce quickly back to the surface outline huge mountain ranges thrusting up jagged peaks, it takes much longer for echoes to return from the deep trenches in the sea floor-the greatest of which, the Mindanao and the Mariana trenches of the western Pacific, form giant gashes reaching down some seven miles below sea level.

In addition to giving oceanographers a picture of the varied shape of the sea floor, echo sounding acquainted them with the deep ocean's true range of depths-which generally lie from 12,000 to 8,000 feet (3660 to 2440 meters) and outlined the rims of the ocean basins. Within the unprobed sea floor existed geological won­ders never suspected until strangely behaving echoes aroused our curiosity.

Oceanographers' soundings have established that the ocean floor is divided into three distinct areas: the con­tinental shelf, the continental slope and the deep-ocean basin.

The continental shelf borders on continental land areas, and some nations have agreed that, in a legal sense, it is a part of the land out to a depth of 200 meters (about 656 feet). Actually the shelves vary a great deal from this idealized definition and sometimes extend out from the continents for hundreds of miles. They vary from flat, terrace-like plains to irregular, rough terrain. A combination of sediments-rocks, sand, mud, silt, clay and gravel-blankets the shelves. Sand forms the most common sedimentary material. It consists mainly of coarse particles eroded directly from the land, transported by rivers, currents, ice, wind and volcanic eruptions.

Biologically the continental shelves arc shallow, sun­lit seas supporting immense numbers and varieties of animal and plant life. The energy of the sunlight per­meating these waters is used in the process of photo­synthesis by a variety of algae and other marine plants, including the minute diatoms. Such tiny free-floating plants form the phytoplankton, the basic "producers" of the sea. These drifting organisms provide photosynthesized proteins, starches and sugars to marine animals.

The drifting organisms are not all tiny plants. The larval forms of many marine animal species, such as starfish, urchins and corals, begin life in the plankton stage. These tiny drifting larvae are part of the zooplankton. The zooplankton also includes the holoplankton, animals that spend their entire life cycles as minute, free-floating organisms. The dinoflagallates are among these.

The continental crust actually ends near the place where the continental shelf drops rapidly to the ocean floor. This sharp descent is called the continental slope, and here the deep sea truly begins. Geologists know that the slopes generally drop at from 100 to 500 feet per mile. They are generally cloaked by sediments composed mainly of mud, a little sand and small amounts of gravel.

In some areas the steepness of the slope is quite dramatic. For example, the drop-off along the western coast of South America from the top of the Andes Mountains to the bottom of the Peru-Chile Trench measures some 42,000 feet, about the same height one would fly in a passenger airliner. Here there is no shelf to break the slope's sharp grade from the coastline to the edge of the trench-the near eight-mile descent occurs over a horizontal distance of less than 100 miles. The steepness of this slope dwarfs any other on Earth; most are much more gradual. Many descend like hill­side terraces in a series of basins and plateaus.

The ocean-floor trenches generally run parallel to the continental slopes. Often the trenches lie next to rows of active volcanoes, and many earthquakes are generated in their vicinity-evidence that the deep-ocean trenches are the sites of powerful geological activity. It is not surprising that the deepest ocean-floor trenches are located along the perimeter of the "Ring of Fire"-the active volcano belt that encircles the Pa­cific Ocean basin.

Geological research suggests that the Pacific Ocean basin is shrinking in area as a result of the movement of continental plates along the edges of the basin. The trenches are chasms where the crustal plates of the ocean floor descend into the Earth's interior as they are overridden by continental plates.

Spectacular canyons are known to exist in the conti­nental slopes. Many scientists believe that undersea "turbidity currents" may help carve such canyons. Turbidity currents are rapidly moving streams of water loaded with sediments. They probably begin as under­water mudslides. Water-saturated material begins to move down the incline of a continental slope. Gather­ing up rocks and gravel, it pours down the slope with increasing momentum, cutting deeply into whatever lies in its path. When it reaches a level area, the cur­rent slows, depositing its load of debris. Geologists believe that the heads of canyons were once above the sea, and have since sunk or "drowned" by the rise of sea level. Turbidity currents may have kept these can­yons cleansed of sediments since their submergence.

The open sea beyond the shelf margin is called the oceanic region. The top layer of water is penetrated by sunlight, permitting plants to carry on photosyn­thesis. The oceanic region, though, supports less life than the shelf area. Most oceanic life is "pelagic" (free-swimming). In contrast to the shelves, few animals are able to live on the ocean bottom.

Larvae are less com­mon in the deep ocean, and the plankton is primarily holoplankton-creatures whose entire life cycles are spent as free-floating organisms. (The European eels that breed in the Sargasso Sea and whose larval stages develop as marine plankton are one obvious exception to this rule.)

As the continental slopes continue to descend, they reach the deep ocean basins where the depth averages 15,000 feet. The deep-ocean basins comprise half of the Earth's surface.

Oceanographers estimate that 90 percent of the Pacific deep-ocean basin is rough ter­rain, as opposed to the smooth "abyssal plains" that are more common in the Atlantic basin. Abyssal plains are believed to result primarily from the undisturbed piling up of sediments by turbidity currents.

Traversing every deep ocean floor is an impressive ridge. The first to be discovered was the awesome Mid-Atlantic Ridge. This huge mountain range, soaring more than 30,000 feet above the adjacent sea floor in some places, extends from north of Iceland to below the tip of South Africa. Peaks rising above the surface create islands such as Ascension Island and the Azores. Between Antarctica and South Africa, the Mid-Atlantic Ridge curves eastward, extending around the world in a 40,000-mile-long mountain chain called the Mid-Ocean Ridge. Many underwater earthquakes occur in a rift running down the ridges' centerlines.

The ocean's "abyssal zone" begins at a depth of 6500 feet, and extends downward to the ocean bottom. There are no "producers"-photosynthesizing plants- in water this deep.

Bits of organic matter filtering down from the sunlit regions of the ocean, as well as dead marine animals falling from above, provide food for the life of the abyssal depths. Life prospers in these dim regions according to the amount of food that rains down from the lighted waters far above.

Bacteria and scavengers transform much of this organic debris into inorganic matter after it reaches the ocean floor. Adapting to their darkened environment, many sea creatures at these depths have modified eye structures. Like the familiar firefly, some have bioluminescent or­gans that create light in darkness.

Besides the sediments that are deposited by turbidity currents on the abyssal plains, there are three other general types of sediments on the deep-sea floor. "Cal­careous ooze," found in warm, comparatively shallow waters, is composed primarily of marine organisms' shells and skeletons rich in calcium carbonate. In deeper and colder waters are found "red clay" sedi­ment, a material that is largely inorganic, and "siliceous ooze," consisting mainly of diatom skeletons (which consist of opal-like silica).

Among the strange and picturesque features of the ocean floor are scattered individual mountains that rise from the sea bottom, but lie submerged under several thousand feet of water today. These isolated peaks, rising a few thousand feet from their bases, are called "seamounts"; seamounts with flattened tops are known as "guyots."

Most guyots are found in three general areas of the Pacific: in a line along the Mid-Pacific Ridge, in another group between the Marianas and Marshall Islands and in a third grouping southeast of the Kamchatka coast of northern Asia, far eastern Russia. The stacking of lava from repeated volcanic eruptions is believed to have created the seamounts and guyots. The guyots' smooth, flat tops indicate that these mountains once stood above the surface, where the action of waves leveled off their peaks.

Something must have subsequently caused the guyots to sink. Geologists think the drowning of the guyots may have involved two processes: the great weight of the volcanic mountains may have depressed the sea floor over the long periods since their creation, while from time to time the level of the oceans was also ris­ing, helping to submerge them.

Undersea canyons, mountains and ridges influence the circulation of sea water. These formations block and channel the movement of deep water and also aid in the stirring and overturning of the seas, greatly af­fecting world climate. But geologists are not yet com­pletely sure how the canyons and trenches are created, nor of the exact part turbidity currents play in under­sea geological phenomena, nor of precisely how the guyots were leveled, not to mention the role of the mid-ocean rifts in the triggering of marine earthquakes. Every year, as new equipment is perfected for explor­ing the deep-sea floor, we learn a little more about these mysteries. Thus we can pierce the secrets of the enigmatic sea and discover in this immense frontier one more clue to the mystery of our evolving planet.