Here is the full list of all 13 Depth 6 achievements.
Complete Depth 6 with the third ending
Locate Depth 7
Pick up 3 blood trees
Capture 20 valid photographs of the enemy IN MOTION
Complete Depth 6 with the second ending
Locate a gas mask
Pick up 6 sticks of Dynamite
Pick up 7 golden coins
Open the secret vault on the 4th depth
Fall from a very high point
Complete Depth 6 with the first ending
Capture 10 valid photographs of the enemy IN MOTION
Locate a fire axe