It first appears in the third season, when a French diver investigating the wreck of a World War II plane on the sea bed discovers the pilot is still alive. The pilot was kept animated for fifty years by the Black Oil - an alien substance that is capable of possessing people. The side effects of possession are extremely unpleasant - the oil leaks from the eyes, and the possessed can fire off bursts of concentrated radiation.

By the time of the X Files film, we learn just how unpleasant the Black Oil really can be. The Conspiracy have been working for years on a plan to turn humans into obedient slaves by spreading the oil through bees infected with a virus. What they didn't realise is that the Black Oil is actually capable of mutating its host into a Grey alien. Once the oil is released, humanity will turn into a giant alien factory.