The Arcitc Sea is the name of the final level in Tomb Raider: Underworld.
The Arctic Sea is split into two sections: The Ocean, and Helheim.
Lara blows a hole in the polar ice, and dives down into the murky depths of the Arctic Sea. In order to open the Helheim Fortress drawbridge, she must first find the missing Helheim Power Stones, and raise the Arctic Sea Statue locks.
Once inside, Lara swims through ruined and submerged tunnels and emerges in a large chamber, housing the gateway into Helheim itself.
Helheim is populated by Thralls and Yeti Thralls. Lara also discovers her mother Amelia in Helheim, who has transformed into a Thrall herself. Lara takes matters into her own hands, and shoots the Amelia-Thrall off a tall structure into a pool of Eitr.
It is here that Natla reveals she killed Lara's father Richard Croft, when he betrayed her in Thailand by trying to sabotage her efforts towards entering Helheim. Natla leaves to 'Raise the Midgard Serpent', and Lara fights her own Doppelgänger, who meets her apparent demise at the hands of Amanda Evert.
Amanda bravely holds off an approaching army of Yeti Thralls, allowing Lara the chance to chase Natla.
In the final large cavernous area lies the Jőrmungandr Mechanism, being primed by Natla to cause global cataclysms and her 'Seventh Age'. Lara uses Thor's Hammer to disable the device, before using it to destroy Natla - sending her flying into a whirlpool of eitr.