Midgard Serpent

"Lara Croft: Shut it down! Jacqueline Natla: Lara! Unexpected, but I'm glad you're here. You should be allowed to see the fruits of your labor. Lara Croft: That's the Midgard Serpent? Jacqueline Natla: Of course not. Jörmungandr is the network of the tectonic ridges that encircles the earth on the ocean floor. And we stand now on its weakest point, where the ancient supercontinent Pangaea first broke in two. When this relic unleashes its fury below, the very seems of the planet will burst. "The Midgard Serpent will rise up and spew poison into the air, and all the world will be consumed by fire and ash." Ragnarök! The seventh age is upon us!"

- Natla reveals to Lara about Midgard Serpent.

The Midgard Serpent or Jörmungandr is a network of tectonic ridges that encircles the earth on the ocean floor. It is the main threat in the game Tomb Raider: Underworld.

In Norse Mythology
Jörmungandr (Old Norse Jǫrmungandr, pronounced [ˈjɔrmuŋɡandr]), often written Jormungand, or Jörmungand and also known as the Midgard Serpent (Old Norse: Midgarðsormr), or World Serpent, is a sea serpent, the middle child of the giantess Angrboða and the god Loki.

According to the Prose Edda, Odin took Loki's three children by Angrboða, the wolf Fenrir, Hel and Jörmungandr, and tossed Jörmungandr into the great ocean that encircles Midgard. The serpent grew so large that he was able to surround the earth and grasp his own tail. As a result, he received the name of the Midgard Serpent or World Serpent. When he lets go, the world will end. Jörmungandr's arch-enemy is the god Thor.

Tomb Raider: Underworld
When the super-continent Pangaea broke in two it left a weak point to the encircling tectonic ridges. It was there at the weakest point where a device was built to activate the tectonic ridges and consume the world in fire and ash.

Natla activated the device to to bring about Ragnarök, the Norse end of the world. Lara Croft's effort to stop both the device and Natla basically plays Thor's role in his fight against Jörmungandr in myth. Lara with the Mjölnir managed to destroy the device and with it the only way to activate the tectonic ridges.

Trivia

 * Lara speculates that Níðhöggr, a great serpent that gnaws at the roots of the World Tree Yggdrasil is another manifestation of Jörmungandr.
 * The device that activates the "Midgard Serpent" has a serpent motif.