Element 115

 is one of the four Meteorite Artefacts Lara was hired to retrieve by Mark Willard.

Origin and history
Element 115, like the rest of the artifacts, was fashioned by Polynesians from the meteorite that had struck the lush continent of Antarctica hundreds of years ago and turned it into a snow desert.

It was taken by one of the sailors from the HMS Beagle to North America. Later, it was acquired by the U.S. Government and placed at Area 51, a remote detachment of the military Edward Air Force Base located in Nevada, where it was used to power an extraterrestrial spacecraft.

Powers
While in the game the artifact doesn't display a visibly power, can be speculated that the Element 115 could work like a powerfull energy source to power any machine or even the other meteorite artifact to amplify its powers.

Trivia

 * Element 115 is allegedly the power source for Alien space craft, reported by supposed whistle-blower Robert Lazar in the late 80s and 90s. Lazar claimed to have worked at Area51 Site-4 where he examined a Flying Saucer recovered by the United States Government. This served as inspiration for the Flying Saucer's main power source in the Area51 level of Tomb Raider.
 * Element 115 has also appeared in the game Call of Duty: World at War, and Black Ops 1 and 2's Zombies mode, as the very energy source that was used for weapon technology, teleportation experiments, and creating the zombies.
 * Additionally, an alien substance known as Elerium-115 plays an important role as primary resource in the X-COM franchise. It also appears to be crystalline in form and is highly sought after (and recovered samples are usually well-protected.)

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