Forcefields are automated defenses found protecting the Alien Lord in the UFO Crash Site in the Ice Caves.
Mechanics
Forcefields discharge vertical arcs of purple energy that kill anything that touches it and reflects any thrown object that collides with it, but not bullets from the Shotgun or Pistol.
Disarming
The forcefield will cease to function if the generator is destroyed, which can be done by throwing an object at it, blowing it up with a bomb or by shooting it with a gun.