Abstract: An underground structure formed of rock and cinders permits an artificial water table to be maintained at a selected distance below the ground level. The installation includes a watertight membrane that lines a basin excavated in the earth. A ballast layer of generally uniform size rock is laid on top of the watertight membrane in the basin, and the ballast layer is extremely permeable by water. The ballast layer is covered by a permeable membrane such as cloth. The permeable membrane is, in turn, covered by a layer of finely divided particles such as crushed volcanic ash or cinders. The particles of this fine layer are prevented from penetrating the ballast layer by the permeable membrane. The fine layer permits water to diffuse through it by capillary action. The fine layer is topped by a finish layer which is appropriate to the use to be made of the installation.