You know? Of course I found that abandoned campsite with the hole, I investigated it right after leaving the forest and then right after finding the rope in the first building I dropped everything to go immediately to see if I could descend... but the game didn't allowed me. Then I went to enter the second building and at that moment I met Red. I guess now that the game was programmed so that the rope thing would only be enabled after the encounter with Red.
I just tried again now that I read your answer and indeed the option to descend has appeared. Thanks for responding and sorry if I bothered you ;D
PS: All this mind control and gelatinous beings, or slime, have you played Changed (DragonSnow)? :3
But it's good that this one does show NSFW content, unlike Changed that tempts you and caresses the subject but never really gets into it... it's as if Changed seduces you with the idea but always leaves you wanting to do the thing.