Percussion tool for underwater working, e.g. for breaking up rocks, pile driving etc., of the known type has working punch element and a hammer element but in which the hammer consists of a hollow cylinder in which the working punch or chisel is at least partially lodged and means are provided for displacing =1 of these elements relatively to the other, and for creating a vacuum in a cavity arranged between these elements in such a way that after their release, the energy accumulated by =1 element as a result of the pressure of the ambient medium is transmitted at least partly to the other element. The tool is less damaging to the environment than the use of explosive charges and has the great advantage that the efficiency increases with depth of use. If the cavity is initially filled with water, this can be driven out by detonating the product of a reaction between water and a suitable matl. e.g. potassium peroxide/lithium hydride, by means of the electrical element.
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