The Shift mod for Half-Life 2

The Shift returns to the setting and timeline of the original story featuring the ubiquitous Black Mesa Research Facility security guard Barney Calhoun. As Barney, the player attempts to escape the alien invasion caused by the resonance cascade and the ensuing military cover-up. The Shift begins in a similar manner to Half-Life, as Barney Calhoun rides a tram through the Black Mesa facility to reach his place of work. After reporting for duty, Calhoun is instructed to assist in maintenance on a malfunctioning elevator. As Calhoun finishes repairs, however, Freeman's experiment takes place and results in a "resonance cascade", causing massive damage to the facility and teleporting alien creatures into the base. The elevator is badly damaged and fails, sending Calhoun plummeting into the depths of Black Mesa.