10 Things We Learned From Twin Peaks: The Return Part 8
6. Experiment = Mother
Within that batsh*t sequence, we see the androgynous figure that eviscerated Sam and Tracy in the two-hour premiere floating in a blank void, arms outstretched (reminiscent of the manner in which Cooper was suspended in the glass box in New York City, having been plunged into "nonexistence" by the Evolution Of The Arm's doppelgänger).
We see this figure - credited, for now, as 'Experiment' eject...something. We are dealing with something literally alien, indefinable by physical concepts. To the human eye, it echoes the mushroom cloud itself, an instantly-fossilised, translucent, frozen stream of grey matter within which there are multiple small eggs (hosting the blackened, hobo-like spectres?) and a sac - unquestionably - bearing the visage of BOB.
In the original series, Philip Gerard/MIKE suggested that (by 1989) BOB had been with us for "over forty years". This sequence is self-evident - BOB's face emerges from an amniotic sac - and the math all but confirms that this is the birth of BOB, the evil that men do.
Therefore Experiment = "Mother": the unseen, door-pounding entity from which Cooper narrowly escapes in the mauve world of Part 3.