Jörmungandr

The Jörmungandr Mechanism, which appears at the end of Tomb Raider Underworld, is a device associated with the Midgard Serpent and whose activation can bring about Ragnarok, the Norse end of the world. The generally accepted role for the Mechanism is that it is some sort of deterrent Doomsday device.

A loading screen for the DLC Lara's Shadow says that;

Jormungandr's awakening has been averted, delaying Ragnarök in the process. But at what cost? The world tree has been decimated and one of the few remaining bastions of Atlantean technology lies crumbled at the end of the world.

The suggestion that the Mechanism was created by the Atlanteans would seem to match up with a comment that Natla makes in Anniversary; "This island is just one remnant of Atlantis; I will find another!"

Jörmungandr, the Midgard Serpent

In Norse mythology Jörmungandr ['jœrmuŋgandr], alternately referred to as the Midgard Serpent (Midgardsormr) or World Serpent, is a sea serpent, the middle child of the giantess Angrboða and the god Loki[1]. According to the Prose Edda, Odin took Loki's three children, Fenrisúlfr, Hel and Jörmungandr. He tossed Jörmungandr into the great ocean that encircles Midgard. The serpent grew so big that he was able to surround the Earth and grasp his own tail. Jörmungandr's arch enemy is the god Thor.