One week earlier, Lara was on her boat floating on the Mediterranean sea, as the cutscene shows. Using her laptop and speaking to Zip and Alister, Lara says she is close to finding Avalon. Her father was convinced that the path to the undiscovered island was here, as Professor Eddigton* told her. Although Alister doubts that her mother is still alive, but Lara wants to find the truth.

[*Note: Underworld follows the storyline of Legend (at the end of the game Lara asked Zip to call Professor Eddigton and arrange a meeting) but it also includes elements from Anniversary (the remake of the original Tomb Raider game). If you have not played those games, you may have troubles following the story. To get an idea of what has happened, you can select Extras from the main menu, Trailers & Credits and then Previously On Tomb Raider.]

You begin on Lara's boat. This part of the level takes place underwater, but you will not have to worry about your breath because Lara wears an oxygen tank.

At the bottom of the sea - Jump into the water and swin down. Keep swimming until you reach the bottom, where you will be able to see the rock structures. As you you explore, you will encounter three sharks. They do not attack all together, so keep your eyes open.

(Note about the guns: Lara can use her Pistols underwater, but it is more effective here if you use the Spear Gun. Enter the PDA, select Weapon Selection and choose the Spear Gun. Lara has now instant access to two guns: the Pistols and the Spear Gun. If you want another weapon, you can select it from the PDA and it will accompany the Pistols.)

The door puzzle - Swim towards the tall rock structure. The entrance is at the bottom, so swim through it. Be careful of the jellyfish, because they can hurt you. Continue to the room below where you will find the first puzzle. This is a door that consists of three mechanisms. Each one needs a handle to work and the middle mechanism has already the handle in its place, so you are missing the other two. The First Handle is on a pedestal at the bottom of the room. Get it and approach the center of one of the two mechanisms. Press the Interact button to put the Handle in its place. Do not rotate any of the Handles yet, so it will be easier later on.

Turn around to face the entrance and locate the tunnel above to the left (as shown in this screenshot). Swim through it until you reach almost the top. Look around and you will spot the Treasure (1/26) on a rock. Continue to the top of the tunnel to reach the sea. Turn right and swim to the front side of the tall structure. With the entrance to your back, locate the small cave with the rectangular entrance to the left (as shown in this screenshot). Swim through it and again avoid the jellyfish. Follow the cave as it goes around to the right and swim through the opening. The Second Handle is on the right (see here), whilst on the left is a path above some rocks that leads to a Treasure (2/26).

Solving the door puzzle - Make your way back to the tall structure and return to the door puzzle. Put the Second Handle in its place and examine the door. Each mechanism has three almond-shaped pieces. Notice that the middle mechanism has one common piece with the left and one common piece with the right mechanism (as shown here). You have to rotate the mechanisms so that the middle one has the three pieces with the "opened eye" designs (as shown in this screenshot). If you have not used the Handles at all, this is how you can open the door: Use the middle Handle once, the left one once and the right one twice. Once you do it correctly, the door opens.

If you want all the Treasures, do not swim through the door (if you are not interested, skip to the part of the walkthrough Past the door puzzle). Turn around and exit the tall structure. Swim to the left (having the entrance to your back) and look for some fallen rocks that create an archway (as shown here). Swim towards it and you will see an opening. Follow the tunnel and you will spot the Treasure (3/26) beneath the jellyfish. Get it and return back to the sea.

Facing the entrance of the tall structure, there is a cave on the left, but the entrance is hidden behind the seaweed (as shown in this screenshot ). Swim through it and follow the cave as it goes around to the right. In the next room, you will see an empty pedestal on the right. To the left is a way above the rocks that leads to the Treasure (4/26). Once you get it, return back to the sea.

There are more fallen rocks to the far left of the tall structure (behind the cave of the fourth Treasure - see this screenshot to get oriented). Swim behind the rocks and you will see an opening. Get into the cave and follow the tunnel to reach the Treasure (5/26). Return back to the sea.

Approach the front side of the tall structure and swim upwards. Almost halfway up, look for a flat ledge on the left where the Treasure (6/26) is lying on. (This screenshot shows the location, but if you want a close up, see here).

Past the door puzzle - Once everything is in your backpack, swim through the door where you solved the puzzle and follow the tunnel as it goes, being careful of the jellyfish. When you reach the third corner, turn around to see the way you came. Look down and you will see another Treasure (7/26) on the right (as shown in this screenshot). Continue to the end of the tunnel that seems to be a dead end. When you approach the giant tentacle, it will retreat and clear the way. Swim through the opening and surface above.

Climb out of the water on the low ledge and climb into the opening. Break the vases on the right; one of them contains a Treasure (8/26). Continue to the other side of the cave and climb up the ledges. Drop down at the other side and go through the opening. Jump into the water and swim down to get a Treasure (9/26) behind the ascent. Now go up the ascent to reach the ground. You need to reach the opposite side of this room where the exit door is. On your way, you will get a Treasure and there are two ways to do it, but first get the Health Potion from the ledge on the left (if the pool is to your back).

First way - From the ledge where you got the Health Potion, jump up to grab the handhold above, then jump to grab the upper ledge. Pull up and move to the right (Lara's left hand). Jump to grab the handhold of the pillar and traverse around the corner. Jump to grab the handhold above, the jump to grab the horizontal pole behind you. Swing and jump to grab the second pole, then the third one. From there, jump to the square ledge ahead where you can break the vases and get a Treasure (10/26).

Second way - From the ledge where you got the Health Potion, drop down and go up the ramp at the other side. Climb on the ledge to the left and jump to grab the handhold of the pillar. Traverse to the right around the three corners. Jump up to grab the handhold above and jump back to grab the square ledge behind you. Pull up, break the vases and pick up the Treasure (10/26).

Turn around and jump to grab the handhold of the pillar. Traverse to the right around the two corners and jump to grab the horizontal pole. Swing and jump to grab the handhold of the opposite pillar. Traverse around the corner and release twice to grab the handholds below. Then drop to the ground.

The serpent puzzle - If you use the switch on the platform now, nothing will happen. You will see only sparks in the two recesses, indicating that fires will lit. To make them work, you have to do something with the two sets of pressure plates on the floor.

Locate the two small blocks between the plates. Pick up the first one (stand in front of it and press the Interct button), go to the left plate and press the Crouch button to set it down. Do the same for the second block, putting it again on the left pressure plate. Both will have enough weight to keep the plate down.

Now stand facing the right serpent and use the Grapple to cath the ring on the head. Press the Interact button to pull it and make it fall on the pressure plate. Now use the switch on the platform to operate the mechanism.

The stone door opens in the cut scene and Lara finds the mechanism incredible. As she examines the carvings, she finds them to be early German design, but older than the 5th century. She spots the runes above the door and reads "World of Mist" and she takes it to be Niflheim, the realm of the dead; not Avalon, but the Norse equivalent.