Project Imagination Doorway, also known as Project X, was an experiment to use the imagination against the Soviets during the Cold War. However, the United States government was unable get the portal to open until the events of the "Imaginationland" trilogy.
The portal itself can only be activated by singing the "Imagination Song", an annoyingly long song consisting solely of the word "Imagination" in different tones. The song is described by a government technician as a "multitonal resonance code sequence" required for the "fractal converter" to function. Singing portions of the song will induce an "electrofeedback nanoresponse" in a correctly-calibrated imagination doorway, but only singing the song in its entirety will open a corridor stable enough for passage.
The doorway is strikingly similar to the portal used in the Stargate series. Stargate actor Kurt Russell was drafted to pioneer the exploration of the realm beyond the doorway, on the grounds that he was in "that one movie that was kinda like this".
The name of the project is, ironically, incredibly unimaginative. Later, the Imagination Doorway is used to send a nuclear bomb to Imaginationland in a plan equally unimaginatively called Operation Nuke the Imagination Through the Imagination Doorway.