In the condensation reaction of nitromethane with glyoxal carried out in an aqueous solution of sodium hydroxide, 3,6-dinitro-cyclohexane-1,2,4,5-tetraol was obtained (the expected product, described in the literature) and, unexpectedly, also tricyclic nitro-triol (6b-nitrohexahydro-2H-1,3,5-trioxacyclopenta[cd]-pentalene-2,4,6-triol), which has been unknown until now, was obtained as the main product. The structure of the compound was confirmed with 1H NMR and 13C NMR spectroscopy, LR, and HR-MS techniques and with single-crystal X-ray diffractometry. The tricyclic triol (formally a hemiacetal) was transformed into 6b-nitrohexahydro-2H-1,3,5-trioxacyclopenta[cd]-pentalene-2,4,6-triyl trinitrate by reaction with 98 % HNO3. Some explosive properties of this compound were determined including: friction and impact sensitivity, activation energy, detonation velocity, heat of combustion in an oxygen atmosphere and enthalpy of formation. The nitrate ester is a powerful explosive with performance close to that of pentaerythritol tetranitrate (PETN).