Helheim

Helheim, the Norse Underworld, is actually Xibalba and Avalon. Natla tells this to Lara in the Mediterranean level of Tomb Raider Underworld, and that no one can enter without Thor's Hammer.

Helheim is where Lara's mother actually is. In one of the gameplay trailers, Lara tells Zip what she heard from Natla; "Helheim, Zip. Various myths use different names, but they all apply to the same remnants of the ancient world".

The final level of Tomb Raider Underworld takes place there,where it houses the Midgard serpent.