Science fiction is a literary form which enables authors to imagine amazing and sometimes frightening ideas of what life may be like on other planets or even on earth at some time in the past or future. Sometimes fact is stranger than fiction for one of the most intriguing stories of scientific discovery concerns our own planet, Earth, and the way the continental areas have evolved and assumed their present distribution.

For a long period geographers and geologists thought that there was a strong degree of permanence in the distribution and form of the continents and ocean basins. Although the highest mountain on land and the deepest parts of the sea were of great interest, it was significant that the average level of land occurs at 870 m above present sea level and that the average depth of the sea is 3800 m below sea level. This average picture, shown on the hypsometric diagram (Figure 2.1), also gives a good idea of the general shape of the ocean basins and their resemblance to a soup dish. The ocean waters slightly overfill the dish and extend on to the gentle slope of the rim. This gentle slope, known as the continental shelf, is bounded on the ocean side by a relatively sharp descent to the ocean depths of the abyssal plain. The North Sea and the shallow seas around Great Britain are a good example of the continental shelf at the present time.