We've known that aliens exist in the Fallout universe for a long time now, with crashed spaceships and alien corpses appearing in several games in the series. Heck, you can get beamed up to a mothership in an expansion for Fallout 3, and there's an alien invasion event in Fallout 76.
In Fallout 4 you can find a spaceship and fight a single living alien, but modder RascalArt decided it was time to finally begin a proper alien invasion. In the Zetans—Alien Invaders in the Commonwealth mod (opens in new tab), extraterrestrial visitors are done being coy and let their true intentions be known. The occupation of Earth has begun with flying saucers and alien ground forces.
I can't direct you to a specific place to find the extraterrestrial visitors—the aliens of the Zetans mod seem to pop up as random encounters, just like lots of other vanilla Fallout 4 enemies. I installed the mod and began exploring the world looking for them, eventually coming across an amazing sight. As I was crossing a field near a ruined bridge, a flying saucer slowly rose from the ground, spun around, and began attacking me.
And it was kinda awesome to see! I was so transfixed by the sight of a flying saucer I just sort of stood there as it began strafing me with energy weapons. I couldn't just gawk, though, because an alien soldier also opened fire on me from the ground. When I'd finally killed them both (the saucer took a long time because I didn't have any heavy weapons on me) I got a couple rewards. First, the saucer hit the ground and detonated in a brilliant explosion, and second, the alien grunt I killed had a Zetan pulse rifle on him. Perfect. I'll use their advanced technology against them.
It's not just flying saucers and simple soldiers you'll have to contend with: the Zetan's ground forces have different weapons and capabilities. There are basic Zetan grunts with blaster pistols and light armor, commandos with advanced rifles and cloaking tech, and even rocket troopers who fly around in jet packs and dual-wield pistols. An hour later I even encountered a Zetan overlord, who looked like a space wizard in a long cloak, used telekinetic attacks, and was flanked by two hovering combat drones.
The UFOs come in different flavors, too. There's the standard issue flying saucer and a more durable, heavily armored transport UFO that can beam down squads of alien soldiers for a ground assault. Even after playing a few hours with the mod installed I haven't come across all the different flavors of Zetan yet. I know there are more out there, I just need to spend more time hunting for them. And I'm happy to do it, they're fun to fight and a nice change from the vanilla mutants and monsters you encounter in the world.
The Zetans aren't alone. They've brought some cannon fodder with them, frog-like aliens called froggos who have been forced into the Zetans' service. You may have to contend with mobs of froggos while you're out in the world, and some even stomp around in armored mechs with acid-spewing cannons. On the plus side, you can also liberate one of these cute little guys. A froggo named Frogbert is in the Speakeasy basement in Concord, and you can add him as a follower. Don't forget to pick up his stuff from the nearby duffel bag: It contains some hats, armor, and other items you can equip on him. He's adorable.
And don't worry, the aliens aren't just there to kill you. I came across some fighting a pack of raiders, and in one particularly memorable close encounter, two saucers opened fire on some ghouls who were already battling a swarm of bloodbugs in a swamp. That gave me a chance to crouch down and admire the UFOs as they circled around and blasted everyone to pieces. Unfortunately, then they blasted me to pieces.
If you want to bring an alien invasion to the Commonwealth, and I definitely recommend it, you'll find the mod here at Nexus Mods (opens in new tab). The Zetans mod doesn't need any other mods to get it running, which is great, though it does require Fallout 4's Automatron add-on (opens in new tab) which is conveniently 60% off on Steam until October 18. Not a bad time to grab it if you haven't.