The final report of the Major Incident Investigation Board (MIIB) was written in 2008 and released in February 2011. The investigation found that Tank 912 at the Buncefield oil storage depot was being filled with petrol. The tank had a level gauge that employees used to monitor the level manually, and an independent high-level switch which would shut off inflow if the level got above a certain setpoint. On Tank 912, the manual gauge was stuck and the independent shut-off switch was inoperative, meaning that the tank was being "filled blind" with petrol (i.e., being filled without a clear indication of the level). Eventually Tank 912 filled up completely, the petrol overflowed through vents at the top, and formed a vapour cloud near ground level, which ignited and exploded. The fires from the explosion then lasted for five days.