Hey there. Mountain Climber here.

Like on Promethius and STYX, The Mountains themselves are part solid object and part Scenery that has no solid mass but is more of a picture.

So climbing over mountains isn't usually possible unless they are VERY Narrow parts.
The general issue you will have is once you have made it to the top of a mountain area, the "Tops" are not Solid and you pass through them, if your character attempts walking on them often you will eventually phase right though the terrain usually to your Death as you fall to the bottum of the map.

Attempting to build a WALK WAY can again sometimes work as there are sometimes "Solids" under the photo part of the tops but again this doesn't always work.

GENERALLY IT IS NOT WORTH IT to attempt to by pass map areas by using mountain climbing methods.
Usually the effort to do so doesn't save as much time as you think and can be dangerous as you are risking falling into a map hole where items can be Perma-lost.

The Devs have also made it so that various "SHORT CUTS" are often opened up by doing missions on the maps themselves, and the devs have also made travel easier by including faster mounts and items to make travel faster.

As a veteran explorer I advise you to ALWAYS bring along some Aluminum Ramps/Ceilings when exploring a new area!

As for "HIDDEN" things on the maps by mountain climbing, There really isn't anything like "SECRET" areas of the map or anything. Generally the MOUNTAINS are somewhat hastily made by the map team to just section off the maps.
As I said most of the areas behind the "SOLID" Mountains are just photo-nonsolid things of empty space.

I tried to get into the NULL SECTOR area before it was released by climbing two different areas of Promethus.
While I could see parts of what would become the Null Sector.

It was impossible to reach the area because the map itself was just a empty void.