UFO crash site

 is an unmarked location in the Commonwealth in Fallout 4 just to the east of Oberland station. However, there won't be anything there until the UFO crash occurs. I'm not sure how to trigger the UFO crash, so the best I can do is describe the state of my game at the time that the crash occurred, with the hopes that others can reproduce it too.

Triggering the Crash
Upon fast-travel to Graygarden, long after completing Troubled Waters and with Codsworth as my companion, Codsworth said "What's that noise" and then a reddish flying saucer streaked from north to south, barely clearing the mountain and clearly about to crash.

Layout
Its a UFO half-buried in a hillside with a skid area of broken trees and flattened ground. The trees all around will be permanently on fire, making the location easier to spot from afar. There isn't anything particularly interesting about the crash site itself, but there is a blood trail of greenish blood leading off to the North. Following this trail, you get to an unmarked cave.

Inside the cave is an Alien (Fallout 4) with an Alien blaster pistol and 505 Alien blaster rounds.

Appearances
appears only in Fallout 4.