At 1:34 a.m. on April 12, 1991, an explosion near Sasovo, Ryazan Oblast, gouged a 28-meter-wide crater with no trace of conventional explosive residue, detonator hardware, or blast shrapnel. Witnesses had observed large glowing spheres drifting over the site hours earlier, and a tree ten meters from the epicenter was unscathed.

Military investigators including Colonel Prodan and fire chief Matveyev, both experienced with industrial explosions, stated the blast pattern was unlike any conventional detonation. The absence of nitrogen compounds, detonator hardware, or shrapnel ruled out munitions, mining explosives, and gas. A secondary unexplained explosion occurred in 1992 some nine kilometers away, also leaving a crater. Russian geophysicists proposed tectonic hydrogen degassing as the most plausible mechanism; the CIA FOIA reading room holds a declassified assessment of diverging theories on the cause.