In this paper, it is briefly introduced that China's Ordovician limestone confined-water menaces coal mining and Ordovician water is prevented and cured to secure the safety of coal mining. The theory of "Down-Three-Zones" is presented on the basis of research and practice in recent ten years. The concept of "Down-Three-Zones" of floor water-eruption is described clearly._x000D_
Finally, the principle and method of applying the theory of "Down-Three-Zones" are discussed. 1. An Introduction of Damage: China is rich in coal resources._x000D_
The conditions of coal field geology are quite complicated._x000D_
There is an area of two million square kilometre of Karst limestone in nine million six thousand square kilometres of territory. The resources of underground water are rather plentiful, but this menaces coal exploration. Sixty percent of coal mines in China are damaged by limestone confined-water in different scales, particularly coal mines located the Carboniferous-Permian coal fields of the Ordovician limestone distribution area of Northern China._x000D_
Because there is the 500-800 metre thickness of Ordovocian limestone and the well developed Karst containsl plentiful water, Karst water can have more than 100 Kg/cm pressure._x000D_
Limestone aquifers have only distances of several metres or several decade metres to coal seam, Karst water can enter the aquituge and even the coal seam, large eruptions often occur._x000D_
For example, a mine of 3 million ton production capacity had an eruption of 2053 cubic metres per minute. This is the largest confined-water eruption which has brought enormous economic damage and threatened the safe production of coal mining. Therefore, the prevention and cure of Ordovician water has immense advantage to the development of the coal industry. There are several decades experience in the prevention of Ordovician water-eruption in China. The prevention of Ordovician water-eruption has two kinds of plans. 1) dewatering and depression: it refers to dewatering of aquifers as roofs of seam and depression of aquifers as floors of seam in mining areas. 2) comprehensive prevention under the condition of having water pressure in aquiters._x000D_
Due to enormous water, electricity, equipment and lots of other factors, the former isn't used extensively, the latter tits to China's actual conditions and is used widely._x000D_
So-called comprehensive prevention under the condition of