Electromagnetic wave structure modulation apparatus comprises an encoded device that has an electromagnetic wave-transparent material having a three-dimensional volume and electromagnetic wave-opaque walls, and a plurality of electromagnetic wave-opaque projections projecting into the electromagnetic wave-transparent material. The apparatus further comprises an interrogating device that has a transceiver operable to transmit an electromagnetic wave into the electromagnetic wave-transparent material, the electromagnetic wave being modulated and reflected by the electromagnetic wave-opaque walls and projections, an antenna operable to receive the reflected electromagnetic waves, and a decoder operable to decode the received reflected electromagnetic waves.