After the crash, you can stumble onto the wreckage by heading north of Somerville. Apart from proof of intelligent life beyond Earth, there's not really much to see ... until you notice a green puddle on the ground. If sci-fi has taught us anything, it's that whenever something or someone is bleeding, they'll leave a perfect trail of blood. It leads to a nearby cave, where you'll make first contact with an alien species -- searing hot lead contact right between the eyes. In classic psychopathic RPG player fashion, you have to gun down this injured survivor in cold (green) blood.
This senseless act of violence rewards you with a new way to wreak havoc: the alien blaster. Looking like something you'd see tucked into Marvin the Martian's little green skirt, the blaster vaporizes enemies (and civilians who look at you the wrong way) into bright blue gas. Keep in mind, though, that the ammo you get when you find it is all you'll ever get, so don't come crying to us if you waste it all on radroaches.
If you've logged tons of hours in this game but never saw any of this, you're not alone. Nobody really knows what triggers the UFO crash event. Some say it occurs when your character hits Level 12 or 15. Most say after 20. Some people say you have to clear out Vault 75 first, or Malvern Middle School. Others claim they saw it without ever visiting those places, so for all we know, you have to be playing on a Tuesday after getting a haircut.