Big MT

The Big Empty is a high-tech location located east of the Sierra Madre. It is likely called the "Big Empty" because it is speculated to be the "Big Mt. Training Facility" and was thus renamed by phonetically pronouncing the letters M and T together, as in shorthand for the word "Mountain." Similar coined names for landmarks such as The Pitt, Novac, and Arefu are examples of such improvised naming schemes.

Background
A pre-War military facility, the Big Empty has remained secure ever since the Great War, its scientific secrets and treasures undisturbed, protected by robotic guardians. On the outside, it seems as if it didn't exist, however, if one knows where to look, the place is certainly not empty.

The installation has several medical research centers, where experiments that can only be described as atrocities were conducted. People subjected to them were often damaged beyond recognition by the procedures - some could only hear static, others became gibbering wrecks.

Its security was breached sometime in the 23rd century by Elder Elijah, who barely made it out alive with a supply of high-tech slave collars. He trapped Christine, a Brotherhood Scribe hunting him, in one of the aforementioned medical centers. While locked, she was forced to undergo an experiment that included jabbing electrodes into her head and letting an electrical current run through her, severely impairing her mental capacity. She was saved by Ulysses, who also told her where to look for her mark.

Appearances
Big Empty did not appear in any of the published games, but was to appear in the canceled Fallout 3 project by Black Isle codenamed Van Buren, as the Big Mt. Training Facility, an old military boot camp that after the Great War is being run by Mr. Handy robots. It is also mentioned in the Fallout: New Vegas add-on Dead Money.

Behind the scenes
At some point during the development of Van Buren, Big Empty was merged with the Leavenworth prison into what became the Tibbets Prison, which was still referred to as "Big Empty" by some of the developers. It is currently unknown whether the mention of Big Empty in Dead Money refers to the pre-merge version of this location, or to Tibbets.