1. You may believe that Mount Everest is the tallest in the world, with its 8,848 m (29,450 ft) in altitude. Yet, it was proven that the volcano Mauna Loa from Hawaii is taller by 2,300 m (7,660 ft),
if we measure it from its base on the bottom of the ocean.
2. The mapping of the oceanic bottom revealed oceans are expanding. The Atlantic, for example, grows by 2.5 cm (1 in) annually in the area of its median dorsal. In other areas, the expansion is more rapid: in the eastern Pacific it can reach 12-16 cm (4.8-6.4 in) annually. But the ocean itself does not grow, as the plates forming the bottom of the Pacific subduct in deep oceanic sinkholes.
3. The most surprising discovery was the bottom of the ocean is no older than 200 million years, while the oldest rock in the ground has an age of 4 billion years. Some rocks on the ocean bottom are older than 200 million years, but they are part of the continents - more specifically, of the continental platform.
4. In some areas, like Chile, the platform is no wider than 2 km (1.2 mi), while the North Sea, for example, is entirely located over the continental platform, and, on the shore of Siberia, the continental platform extends 1,200 km (750 mi) into the sea. Most continental platforms are no deeper than 200 m (660 ft). On their edge, a slope goes down to the abyssal planes, which form most of the ocean bottom.
5. The flat and uniform abyssal plains are located at depths of 4,000-5,000 m (13,330-16,660 ft), being made of basalt (a volcanic rock) covered in sedimentary layers. The basalt has a rough, uneven surface, but the sediments smoothen the underwater landscape. Sediments consist in the remains of dead animals and plants, most of them coming from the microscopical plankton. Many of these species have lime shells, that's why most of the oceanic muds are calcareous. Chalk is nothing more than fossil oceanic calcareous sediments.
6. The dorsals are formations stretching through the middle of the oceans and molded by solidified magma, forming young rocks. This is the place where the Earth crust receives new matter.
7. Opposite to the dorsals are the oceanic sinkholes, through which the plates sink and enter back into the composition of the Earth's mantle. Sinkholes are found, for example, near the western shores of South America, that's why the continental platform is so narrow there.
8. Abyssal mountains emerge in the so-called hot spots on the ocean's bottom. In these places, submarine volcanoes form mountains. Most of the islands of the Pacific and middle Atlantic are the peaks of such mountains, that raised over the sea level. Some of these volcanoes are shut off, but others still active, like in Tristan da Cunha and Canary Islands (Atlantic) or Hawaii (Pacific).