A method for reducing compressed air losses when driving underground cavities which are supported by compressed air. The part of the inner cavity face exposed by mining is sealed by a viscous medium after breaking away the earth. With a supply of propellant, the viscous medium is applied, in the form of a spray mist which is directed toward the exposed part of the inner cavity face, in such a way that the mist penetrates the substratum without substantially destroying the exposed face. Particles of rock are thus prevented from being detached from the face which is to be sealed.