We see with 70% of the earth covered with water, in areas far away from water we see barren deserts and in areas on earth completely surrounded by water we see Antarctic climates.

With the 70% amount of water on earth, it is just the right measurement for humans to survive. The Sun's rays cause the sea and those parts of the earth's surface covered with water to evaporate.

The water vapor that is given off rises into the atmosphere and by condensation, forms into clouds. The clouds produce more rain repeating the cycle and giving plants life which in return give us vegetation and air.

For many centuries, humans held totally inaccurate views regarding hydrogeology. Two specialists on this subject [G. Gastany and B. Blavoux in Universalis Encyclopedia] give a edifying history of the views.

"In the Seventh century B.C., Thales of Miletus held the theory whereby the waters of the oceans, under the effect of winds, were thrust towards the interior of the continents; so the water fell upon the earth and penetrated into the soil.

Plato shared these views and thought that the return of the waters to the oceans was via a great abyss, the 'Tartarus'. This theory had many supporters until the Eighteenth century, one of whom was Descartes.

Aristotle imagined that the water vapor from the soil condensed in cool mountain caverns and formed underground lakes that fed spings.

He was followed by Seneca (1^st Century AD) and many others, until 1877, amoung them O. Volger.

The first clear formulation of the water cycle must be attributed to Bernard Palissy in 1580: he claimed that underground water came from rainwater infiltrating into the soil.

This theory was confimed by E. Mariotte and P. Perrault in the Seventeenth century.

"We sent down water from the sky in measure and lodged it in the ground. And We certainly are able to withdraw it. Therewith for you We gave rise to gardens of palm-trees and vineyards where for you are abundant fruits and of them you eat" {Holy Qur'an 15:22}

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