This is unlocked by retrieving all five of the Crypt treasures; here are the locations of the entrances:
Crypt One: Take the zipline down from the Installation Vista camp. Next, head down the slope, and about halfway between your landing zone and the dead bodies at the end is a small rocky platform, just over to your right. Head up it and squeeze through the crack in the cliff, then break the crumbling wall inside the next chamber.
Crypt Two: from the Hidden Refuge base camp, drop down the cliffs in a southerly direction towards the collapsed bridge. When you get to the small clearing you'll find a cave entrance. Destroy the crumbling wall.
The Lost City: at the end of an underwater tunnel beneath the bottom of the zipline from the first trebuchet. Slide down it, drop off the building into the water and swim through the submerged house.
Crypt One: From the Ruins Encampment, head up the steps to the right, go through the broken archway, and then bear right between the two broken buildings. Follow the path towards the pine trees and then turn left to find a cave in the south-east corner of the map. Break down the crumbling wall at the far left of the chamber and head inside.
Crypt Two: From the Ridgeline camp, head south down the path to the small waterfall. Next, stand on the edge and jump to the small outcrop to your left. Hang from the edge and drop down to the lower outcrop (be careful with your aim -- it's easy to miss), then jump to the next one along, then jump to the waterfall and grab the rockface with your axe, clamber up, head into the cave, and smash down the wall.
Credit to Mat Hall
Guide source: http://www.eurogamer.net/articles/2015-11-09-rise-of-the-tom...