During olden days, it was believed that the land beneath the ocean is slightly sloped. It was also not very easy to measure depths or explore the underworld. Today there are ways to measure exact depths using ultrasonic pulse transmission.
Ultrasonic pulses are transmitted from ships. They travel quickly to the floor of the sea and then bounce back. With the calculation of time the pulses take to bounce back, exact depths can be measured. Today there are even techniques to lower cameras to view the wonders on the sea floor.
It has been found that the large expanses of the ocean floors are indeed flat and not slope. This flat ocean floor is called abyssal plains which accounts for the flattest areas of earth. These plains are covered with thick deposits of sediments that have slowly come down from the waters above. Sediments also get deposited from the shells of marine creatures dwelling in the ocean.
Abyssal plains account only for some 10% of the ocean floor. Elsewhere the ocean floor is as varied as the surface of continents. The ocean floor could be dotted with great volcanoes, crossed by mountain ranges and trenches created by nature due to erosion by rivers or movement of plates under the earth. In fact there is an interconnected mountain system that extends more than 40,000 miles through the Atlantic, Pacific and Indian Oceans.
Volcanoes known as sea mounts are also scattered across the ocean basins. There are submarine mountains called guyots that have flat table like tops. Their peaks were sheared off by the waves at a time when the mountain tops were near the surface of the sea. The sagging of the ocean floor lowered them far below the sea level. The abyssal plains are characterized by complete lack of light, high water pressure, low nutrients and chillness.