Hate to say it but the map page has me feeling like I must be an idiot because I have almost no ability to visualize how the first few areas are connected. Once in the cave proper things seem to make sense, but the description of the drop site, cavern, antechamber, and tunnels has me staring at the page wondering wtf I'm doing wrong in the way I'm trying to interpret the relationship between these locations. With the image seeming to make things more confusing rather than less.
Viewing post in The Horror on Tau Sigma 7 comments
Bullet 1 for the cavern is the main explainer for the layout of the upper caves:
"A large outcropping on the planet’s surface, visible from the drop site. Rounded holes in the smooth rock face give access to a hollow interior. The stone forms chambers, flowing in arches and loops, with a main atrium at ground level dividing into passages and sub-chambers overhead that can only be accessed by climbing."
So, according to this, from the drop site the upper cavern can be accessed from the numerous holes in the hollow outcrop that conceals the cave system. These all lead to a large central atrium. To access the upper tunnels PCs need to climb up from the atrium or find an above ground level hole higher up on the outcrop and climb down.
Hope that helps clear things up.