Exit the Bunker | 5: Just Keep Moving Walkthrough Tomb Raider Guide

Go back to the main area of the bunker and use the newly acquired pry axe on the north-east door, pictured above (rapidly tap the action button); this will unlock a passage to the other side of the forest.

Make your way through the flooded corridors of the bunker, and after a while you'll reach a ladder which will take you to the surface (leap up to reach it). You'll get another objective once you're outside (reward: 100 XP).