There are sixe Cave locations that serve as a transition zones.
The first links Pleasant Valley and Winding River (the latter is itself a transition zone to Mystery Lake); the second links the Mountain Town region and Mystery Lake. A third connects Mountain Town and Hushed River Valley. The fourth links Forlorn Muskeg to Bleak Inlet with some verticality (climbing/ducking ...). The fifth links Timberwolf Mountain and Ash Canyon (via the second entrance located in Deer Clearing). The sixth connect Timberwolf Mountain to Blackrock's prison region.
Pleasant Valley to Winding River
The long and winding tunnels coupled with the low light make navigating this cave difficult. It is highly recommended that players venturing into the cave have a light source such as matches, a fueled storm lantern, or several flares.
Mountain Town to Mystery Lake
The cave has a relatively linear layout. One end is found in the south of the the Mountain Town region, and accessed via the climbing spot at the Picnic Area. The other is found southeast of the Trapper's Homestead in Mystery Lake.
Mountain Town to Hushed River Valley
The second transition cave in Mountain Town is on the northernmost part of the map. Coming from Spruce Falls Bridge going west on the first opportunity, go right follow a slope up farther north and keep close to the rock, and eventually, you will find the cave.
Bleak Inlet to Forlorn Muskeg
A very dark cave where the player will have to stoop at times. It forms a path that splits into 2, only to ultimately reconnect to form a loop. One part of the path forces the player to climb a small cliff using roots; a pine-branch bed is laid out at the base of the cliff. The other path is a straight line accessible by crouching and weaving through a narrow passage. This cave connects to the lower elevations of Bleak Inlet and to the slightly higher elevations of Forlorn Muskeg, not far from the Spence Family Homestead.
Timberwolf Mountain to Ash Canyon
Timberwolf Mountain to Blackrock
The entrance to the cave in Timberwolf mountain can be found by following the frozen river uphill which is right next to Andre's Peak. A Rose Hip bush can be found right in front of the cave. It is a very dark cave and requires an artificial light source. It contains two small cliffs with roots that can be climbed. The lower part of the cave may also contain a ravaged wolf carcass very close to the exit towards Blackrock. Caution is adviced since the cave also contains pools of cold water that can wet the players clothing, though some of them cannot be seen. Whether this a bug or an intented feature is unknown.