When traveling around the Commonwealth, the Sole Survivor hears the sound of an incoming vessel. The UFO will then pass quickly overhead before crashing in the distance. It leaves a distinct trail of smoke to follow to the crash site.
The site features a half-buried UFO in a hillside with a skid area of destruction including broken trees, flattened ground and scattered flames. The surrounding trees are on fire. There is no way to enter the UFO, nor anything to scavenge at the crash, but next to the still burning UFO is a trail of green blood. The Sole Survivor can follow the trail slightly north, down into a ravine and then into a nearby unmarked cave.
The UFO appears only in Fallout 4.