UFO crash site

The crashed UFO 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.

The crash happens randomly for everyone. Fast traveling to Cambridge Police Station with a high enough luck skill, increases the chance the event will happen upon arrival. Visiting Vault 75 may make this event more likely to occur. The event also happens often upon fast traveling to or leaving the Old North Church. Moving over the bridge from the USS Constitution may also cause the event to trigger. Entering Boston Common from one of the nearby indoor areas may help your chances as well.

Layout
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 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 (no way to enter the UFO, nor anything to scavenge at the crash) 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 with an alien blaster pistol and 400-500 alien blaster rounds. The alien will be hostile. It sits near a skeleton and a Ham radio, which is broadcasting in something that resembles an alien language.

Appearances
The crashed UFO appears only in Fallout 4.