• YouTube: Tomb Raider: Underworld - Arctic Sea: Hielheim Part One (Time: 9:11) | 2

1. Use Mjolinir to ward off any sharks. Observe the row of giant statue knights in front of the fortress. For each one that has their sword lowered, you will need to swim into a chamber through the back of their "heads" and turn a crank (three total). TREASURE #1: In the trench separating the main fortress from the row of giant knights. TREASURE #2: Also in the trench, opposite end of Treasure #1. TREASURE #3: In a cave in the trench, right across from the whale bones.
2. One of the knights (in front of the fortress) is missing two gears and cannot lift its sword yet. Swim around behind him to find and pick up the gears on the ground. Swim over to the knight's gear works and set them in. Turn its crank. TREASURE #4: Inside the ruins of the headless knight statue in front of the fortress
3. Next is to seek four red disk-shaped "power keys" to be used on four nodes on the fortress, which will allow you to open the drawbridge in the center. Enter the left-side tower (facing the drawbridge). Swim straight ahead and up into the hole in the ceiling and surface. TREASURE #5: It's at the base of the fortress itself, near the tower.
4. Jump from the slab to the ledge, to the first beam, to the second, then to the ladder. Climb up to the top, then just press A to have Lara jump to the platform with the power key. Get it, and drop back into the water through the smaller hole near the key. TREASURE #6: In a pot near the power key.
5. Dive and swim towards the exit, turning left into the unexplored hallway. Turn left at the fork and swim up into a new hall. TREASURE #7: Instead of turning left, turn right first and collect the treasure wedged in the rubble in the dead-end.

Follow it down to the end for the power key, then swim back out of the tower. TREASURE #8: Right next to the power key. TREASURE #9, #10: There is a path upwards in the ceiling of the hallway leading to the power key. Swim up and look around for both treasures.

1. Swim to the two nodes nearest the tower and insert the keys. Pick up the power key lying here next to the nodes, then go the right-side tower and enter it. TREASURE #10: There's a broken window in the fortress high above the right-hand nodes, facing the tower. Swim in for the treasure. TREASURE #12: At the base of the fortress, near the right-side tower.
2. Surface inside, and climb the ladder. Use the ledge to make your way clockwise around the room to a beam. Jump to the next ladder from the beam. Climb all the way up to the top, then press A and Lara will automatically jump to safety. TREASURE #13: In a pot in the right-side tower, right after you surface inside.
3. Jump from the top of the next ladder to a beam, then to another beam, then up to a cliff containing the power key. Best way down is to drop from Treasure #15 to the long beam, then to the platform below, then to the water. TREASURE #14: In a pot next to the power key. TREASURE #15: In a pot on a small ledge near the power key. Jump to it.
4. Swim out of the tower and use the last two keys on the nodes nearby. Now turn the crank above the drawbridge to open it. TREASURE #16: Right behind the two nodes.
5. Enter the fortress through the open drawbridge and swim down the halls, through the holes in the blockages. Take a right at the fork and grab the health pack, then continue. TREASURE #17: At the first fork, swim straight, and go through the hole in the rubble for the treasure.
6. Swim up the stairs, make a left at the fork and up the next set of stairs. Turn right at the top and swim through the hole in the rubble. Make a right and swim straight through the round gap in the blockage, up the crumbling steps. TREASURE #18: At the first fork, swim right. There is a tiny crevice above the pile of rocks, swim through and get the treasure. TREASURE #19: After the second set of stairs, turn left and look again for another very small crevice in the rock pile that leads to treasure. TREASURE #20: After swimming through the hole in the rubble after the second set of stairs, turn around and pick up the treasure lying next to the rubble. TREASURE #21: Before entering the round gap, swim through yet another small crevice in a rock pile to the left for the treasure, straight ahead from Treasure #20.
7. Through the round gap, turn left to finally surface out of this area. Walk forward for a cutscene. TREASURE #22: Swim straight ahead from leaving the round gap into one more tiny crevice with a treasure beyond it. TREASURE #23, #24: After the cutscene, search the immediate vicinity for two pots with treasure. RELIC #6, SIGIL 1: To the immediate left of the entrance to this area is a sigil. Activate it if you like, but take note of it.
8. Drop down to the platform below and spam overhead smashes with Mjolinir until all the baddies are dead, then climb up the tilting column nearby. TREASURE #25: In a pot right near where you landed. TREASURE #26: In a pot you can reach by walking across a fallen column lying in the blue muck. RELIC #6, SIGIL 2: Take note of the sigil next to Treasure #26.
9. At the top of the column, face Lara towards the small column poking out of the muck and press A. From there, jump towards the slab in the muck with the grip, then scale around to the very end and leap to the next column, then to the platform above with the treasure. Climb down using the column TREASURE #27: Can't miss. TREASURE #28: In the pot next to the column you climbed down. TREASURE #29: From Treasure #28, leap over to this treasure. RELIC #6, SIGIL 3: Note the sigil next to Treasure #29.
10. Jump to the fallen column and use it to climb to a slab, then to safety. Walk forward and get a cutscene.

• YouTube: Tomb Raider: Underworld - Arctic Sea: Yggdrasil (Time: 6:19)

1. Search around for a health pack, then continue. TREASURE #30: By the exit to this area. RELIC #6, SIGIL 4: A final sigil is found to the left of the exit. If all four sigils are in an "activated" state, the waterfall near the grapple point above Treasure #27 will deactivate. The sigils are timed; it's best to start from Sigil 4 and work your way backwards. When the waterfall is shut off, return to Treasure #27's point and grapple to the alcove the waterfall was blocking for the relic.
2. Enter the passage and smash through the armies of Thralls with overhead smashes from Mjolinir. Grab the health packs you find. You will have a cutscene, then down another hall for more smashing. You will eventually arrive at the final "boss" (which is actually an elaborate climbing puzzle).

• YouTube: Tomb Raider: Underworld - Arctic Sea: Out of Time (Time: 8:48)

1. Keep an eye out for health packs at all times. They often hint where to go to next. Run to the right and jump on the grips. Shimmy round. When the slo-mo starts, jump to the left. Jump and scale up the footholds.
2. At the top, jump up to the grip and shimmy left. Jump left when you cannot go further. Shimmy onward. When slo-mo starts, drop down. Follow the footholds and try to jump up towards the next set of footholds. Continue left to a grip that leads to a ledge. Jump to the bridge.
3. Ignore the Thralls and run/roll right past them. Use the grapple point to cross the gap. Reach the end of the line, kill any nearby Thralls, and push the blue block forward.
4. Run back along the bridge until you see a sloping catwalk on the left with blue sigils. Jump onto it and slide down. Grab the health pack here.
5. Jump onto the flying platforms and jump off when it reaches the tower the Thralls destroyed in the earlier cutscene. Jump across the gap with the slick slope and move to the small platform further below. Jump into the wall across; do not wall jump, instead, let Lara fall down onto a grip. Shimmy/jump left and drop down.
6. You should be amongst a ring of platforms underneath the main structures, overlooking a vortex of blue muck. Watch out for Thralls and leap your way to the left to another blue block to push.
7. Continue to follow the path of platforms to the left. There will be another blue block to shove into place. Do so. Then continue onward, back at the area you came from. Scale back up with a big chimney jump, and get back on the flying platforms.
8. Ride them back to the base of the slope with the blue sigils you previously used. It's actually part of the Midgard Serpent itself, feeding it power. Press Y in front of the glowing blue power node and Lara will destroy it with Mjolinir. New flying platforms will be created, but you won't need them right now.
9. Return to the same flying platforms you used and ride them over to the building you first climbed onto upon arriving to this entire area (the stairwell leading into this area should be visible in the background). Jump the health pack lying in the crevice inside the structure, then chimney jump up to the grips.
10. Follow the grips to the top, exactly as before. Up top, run down the path again to the grapple point you had used previously.
11. This time, instead of using it to swing across the gap, scale it upwards and grapple wall run to the ledge holding the health pack.
12. Climb to the top and from there, jump onto the flying platforms and ride them to the next Midgard Serpent tower for Lara to destroy another power node (the structure will resemble the last one completely).
13. Use the grapples to scale down to the lower floor, and jump onto the new flying platforms created by destroying the previous power node. Climb onto the higher-flying platforms until you reach the set that swing close enough to the center structure (the one resembling a coliseum) to jump onto its grips.
14. Follow the path of grips up, being careful with the moving ones. At the highest point, slo-mo will kick in and you must perfectly time a jump up so that you land on a passing grip.
15. Drop down from the grip when it passes over another slope with blue sigils; slide down and again smash the power node there apart. This will complete the game and you will receive the Arctic Sea Expedition achievement. Enjoy the ending.

• YouTube: Tomb Raider: Underworld - Ending (Time: 4:52) | Credits

When the credits finish, select "Treasure Hunt" from the menu and save your game, in case you've missed treasure or relics and would like to earn them. You can select any level to play at any time using the Revisit Location option in the PDA.