The principal product of the reaction of dilithiopentalene (3) with methyllithium and methylene chloride is shown by difference-FT-NMR. spectroscopy to be tetracyclo [4.3.0.03~5.04,6]nona-l, 7-diene (8). This laterally bridged benzvalene derivative explodes violently at temperatures as low as -40° when concentrated. Controlled rearrangement in diluted solution gives indene (4). The key step in the formation of the tetracyclic C9H8-compound 8 is suggested to be an intramolecular 1,4-carbene addition.