The structure, energetics, and bonding of the simplest carborane, H2C(H2)BH2+, is determined by high level ab initio calculations. The molecule is bound by 35.5 kcal mol-1 with respect to the lowest energy dissociation products, CH4 + BH2+, which are produced without a dissociation barrier (i.e. no reverse activation energy) along the minimum energy pathway. A detailed analysis of the occupied valence orbitals shows that the bonding arises from three-centre-two-electron bonding through an unusual carbon-hydrogen-boron (CHB) bridge bond explaining the strong binding energy and the absence of a dissociation barrier. The possibility that CHB bridge bonds may play a role in CH sigma bond activation is discussed. Harmonic vibrational frequencies, infrared absorption intensities, and frequency shifts occurring for 10B, 13C, and D isotopic substitution are reported.