What causes the tides?
Tides are caused by the gravity of the Moon. As the Earth spins round, the water in the oceans is ‘pulled’ towards the Moon slightly, making a bulge. There is a corresponding bulge on the other side of the Earth. Wherever the bulges are positioned it is high tide. In between the water is shallower and so it is low tide. High tides occur every 25 hours, because at the same time that the Earth is spinning on its axis, the Moon is travelling around the Earth once every 27 
days. This means that high tide is about one hour later every day.
What is the continental shelf?
The shallow underwater plateaus of land surrounding the continents form the continental shelf. They slope gently down to a depth of about 180 m, after which they slope more sharply towards the deep ocean. The continental shelf can extend out from the coast for long distances, or it may be narrow, for example off the coast of Chile where the Andes Mountains plunge down into the Pacific with hardly any shelf. Usually the shelves are wider off low – flying regions. The continental shelf is a valuable resource, as most fish are found here. Also, there are plans to mine some of the minerals on the sea – bed in these shallow waters.
How deep are the oceans?
The depth of the oceans varies widely, but the deepest part found so far is called the Marianas Trench. This trench is a deep cut in the bed of the Pacific Ocean, and its depth is as much as 11,034 m. Even at this enormous depth, signs of marine life have been seen in the trench. In 1960 Dr Jacques Piccard and Lt Donald Walsh reached a depth of 10,911 m in a US Navy vessel, Trieste. The Pacific Ocean is home to nine of the ten deepest sea trenches. The exception is the Puerto Rico trench in the western Atlantic Ocean, which has a maximum depth of 9,220 m.
Why did sailors fear the Sargasso Sea?
Sailors feared that the Sargasso Sea was haunted by sea monsters and filled with seaweed that would entangle their ships and prevent their escape. The Sargasso Sea is a patch of ocean in the North Atlantic, about 3,200 km west of the Canary Islands. The ocean currents in this area are very slow, although they are surrounded by fast moving currents. This means that any seaweed floating in the Sargasso Sea will not be carried away. Patches of floating weed can be as large as 30 m in diameter. Despite the legends, they are never thick enough to entangle a ship.
