During experiments to detect carbon chain molecules in the laboratory, an electrical discharge through a dilute mixture of silane and diacetylene was found to produce a molecule of unusual structure that is composed of only silicon and carbon. On spectroscopic analysis this molecule has been shown to have the elemental formula SiC3, a closed-shell singlet electronic ground state, and a planar rhomboidal geometry that consists of atoms in the shape of a distorted four-membered ring with a transannular carbon-carbon bond. Rhomboidal SiC3 is isovalent with rhombic C-4, a long predicted low-lying isomer of C-4 of similar structure which has so far eluded spectroscopic detection. Strong lines of SiC3 can be observed under a wide range of experimental conditions, suggesting that this molecule is quite stable, and that kinetic and thermodynamic factors favor its formation. Radio emission lines of SiC3 have now been detected in space in the molecular envelope of the carbon-rich star IRC+10216.