Soft spot between realities / Judy / under the moon
Okay - so we know that the location at 430 is a crossing point into the alternative reality. The area close the the White Lodge portal (and Jack Rabbit's Palace) on Blue Pine Mountain seems to be another 'soft spot' between the two realities.
This soft spot also seems to be a place that Judy/Experiment/Mother has influence over. Major Briggs's message for Bobby (episode 9), Hawk's map (episode 11), and the Log Lady's final message for Hawk (episode 15) all make reference to 'the one under the moon on Blue Pine Mountain'. The consensus seems to be that this image/entity (also seen on Dark Cooper's playing card) is Judy/Experiment/Mother.
Did anyone else notice that Cooper and Laura were walking in this area on a moonlit night when she disappeared (i.e. under the moon on Blue Pine Mountain)? As other people have pointed out, we also hear the sound played by The Fireman in episode 1 - which is also thought to signify Judy/Experiment/Mother.
Carrying soil in your pockets (as done in episode 14) may be a way of staying grounded within the main reality, if you are walking in this area - to reach the White Lodge portal for example. Neither Laura nor Cooper were carrying soil in their pockets there in episode 18, when Laura is pulled into the alternative reality.
If Judy does have an influence over this area, then it would explain why Cooper needs to find another way into the alternative reality. If this area is a 'soft spot', it might also explain why Cooper was able to access the Black Lodge from that location - rather than the usual Black Lodge portal/entrance at Glastonbury Grove. It also makes sense that Judy would want to have an influence over that area, given that her enemies would pass though there on the way to/from the White Lodge.