A process for micromachining arbitrary structures with high aspect ratios in bulk silicon utilizing standard microelectronic processes is presented. It is based on electrochemical macropore formation on n-type silicon in electrolytes containing hydrofluoric acid. Very regular pore arrays with pore diameters and distances in the micrometer range and pore lengths of several hundred micrometers can be produced with this technique. Wafers with suitable prefabricated pore arrays are used as substrates for a micromachining process including anisotropic etching of very deep structures yielding straight walls to depths of as much as 150 mu m.