One week before the prologue, Lara is out in the Mediterranean Sea.
Jump into the water and swim down. There is danger from two sharks in the water, which tend to approach by the time the game tells you about the tutorial to shoot or lock on enemies. While in water, the pistols tend to be short range.
Head to the bottom. There are a total of three caves that can be explored in the sea. The large structure seems most obvious, and can be entered at the base. Swim inside, and you will see a closed door with symbols but missing handles. The first handle is in plain sight, being on a pedistal, and may be easily put into the door. However, you will need a second handle found in one of the samller caves.
In the face of the large underwater spire and slightly to the left (positioned near and slightly right when you jump off the boat), there is an underwater cavern. The remains of the temple contains a handle, which you can collect and return to the large puzzle door. If you're attacked by sharks, you can attack them and automatically drop the handle - make note where it lands, or check its location in the sonar map.
To solve the puzzle, you need to rotate the segments until the center upper wheel has the three open-eye carvings. If you haven't moved any of the pieces, you can solve this puzzle by turning the center wheel once, the left wheel once, and the right wheel twice. When the segments are in position, the door will open.
Doorway to Niflheim
Beyond the door is a long, non-branching corridor that eventually ends in a large tentacle that retreats. Follow it to surface on what appears as dry land. Climb the nearby ledge, and you will approach the entrance to a pool, and a closed door leading to Niflheim.
You will need to climb a ledge by heading to the corner of the room. There is one general path which lets you cross to the other side, which finishes with a horizontal swing bar and three ledges that you can use to climb down.
To open the door, you need to weigh down to pressure plates to allow the flow of water into the sides. First, use the grapple to pull down a statue on the right-hand side. Next, locate two stone blocks, and place them on the left pressure plate. When both plates are in position, you can pull the middle switch to light the two flames.
There is one relic, and 26 treasures in the Mediterranean Sea. There are ten treasures that can be found in this section.
The first six may be found in the open sea:
- In the puzzle door room, turn right and swim up. There is a treasure in the rock shelf.
- In the second handle room, the treasure is found beyond the handle.
- In the open sea, there is a second cavern you can explore appearing as a medium-sized spire, which contains one treasure.
- Picture the large underwater spire as a face. Circle one-quarter clockwise at eye-level to find a treasure.
- There are slabs further beyond the cavern with the empty pedistal. The treasure may be found among the smabs.
- On the opposite sire of the medium-sized spire is a collapsed arch. Treasure is found under the arch.
Past the puzzle door, four more may be found:
- After the puzzle door, and over some rocks, turn around to see the treasure in the pile of rocks on the right.
- After surfacing after encountering the octopus tentacle, smash the jars on the right to retrieve a treasure.
- In the ledge-climbing room, enter the pool and look in the far left corner.
- In the ledge climbing room, there are two jars on a platform. This is at the first corner that you climb, right at the top.