While the opener to Rise of the Tomb Raider featured a tomb in the main story with the Prophet’s Tomb, the rest of the tombs are mostly side quests that are found within the different regions of the game.
When walking around and nearing one in Rise of the Tomb Raider, a message will always pop up on the screen that says “Optional Tomb Nearby – Explore to find the entrance.” Some of these are a little more obvious, while others may be a little more tricky to find.
The first of the optional ones is one of the easiest to find with the Ice Ship Tomb in the Siberian Wilderness region, more specifically the Glacial Cavern area. You will be first alerted to this one while going through a cave in a very linear area. Upon doing that, follow forward along the platforms and you will get a glimpse of the ship hanging on the wall, though this isn’t where you are going story wise.
Climb up a wall after that and then another, before walking into a cave, with the option to continue out the other side. However, instead stop in the middle area and look to the left, where you should use survival instincts to see a climbable wall ahead.
Leap towards that wall and climb up and you will arrive at your first optional tomb in Rise of the Tomb Raider known as the Ice Ship. For those that still have some trouble finding this one, you can check out the video below that shows the path to the Ice Ship tomb in Rise of the Tomb Raider.