At the start of most games in the hugely popular Fallout series, you’re unceremoniously cast out of your cozy bomb shelter and thrust into the harsh, unforgiving wastelands. But what if you weren’t? What if you stayed underground, tucked away in the relative safety of your nuclear bunker, rebuilding society one room at a time?

That’s Fallout Shelter in a radiation-proof nutshell. Game developer Bethesda stunned the world when it took the unusual step of revealing (and releasing!) its first mobile game during a press conference in 2015. An immediate smash, Fallout Shelter shot to the top of the charts on the back of its brilliant simulation gameplay. And it’s pretty much stayed there.

But even encased in a steel vault, life finds a way to evolve. Here are a few reasons we keep returning to our subterranean safe space:

Rule the roost
As vault Overseer, you’re in charge of a small refuge that grows and develops into a thriving community. It’s your job to build up the shelter, get it running efficiently, and welcome more survivors from the wasteland outside, giving them a safe home and a job in your underground enclave.

It sounds complicated, but your ultimate goal is simple: Keep everyone happy. Putting the right people in the right jobs, keeping them well nourished, and—crucially—ensuring they aren’t eaten by vault invaders like radioactive cockroaches, mole rats, feral ghouls, and imposing deathclaws. You don’t have to be Mr. Nice Overseer, however, as a recent update lets you play bad landlord and evict dwellers who aren’t pulling their weight.

Make some rooms
The more wealth you acquire, the more you can build and expand your vault. Each room yields specific benefits—a classroom, for instance, increases intelligence, while a radio studio will blast your message of goodwill to hopefully expand your community.

Post-launch room updates have added even more features. Keep your dwellers looking snazzy with a barbershop, or deck them out in fancy duds with the new outfit workshop. And if you’d like to unlock Fallout Shelter’s awesome new quest system, you’ll need an Overseer’s office.

Wander the wasteland
It’s a rough world out there, but as Overseer, you can send your hardy little pals out into the not-so-great yonder in search of precious loot. Initially, the poor souls were left to stumble around on their own, but the quest system added a variety of fully explorable locations.

In addition to one-off daily quests, dwellers can partake in multistage missions to track down hidden vaults, solve the lost cases of detective Nick Valentine, investigate alien sightings, and even snuff out the self-styled four horsemen of the post-apocalypse. (You might want to stock up on Nuka Cola before trying that one.) Quests are also a great way to find unique, legendary dwellers, such as Fallout 3’s famous radio DJ, Three Dog.

Fall in love
Let’s face it: Vault life can be lonely. You’re basically stuck in a giant metal box. Not exactly a great setting for romance, but even doomed survivors need companionship. Two dwellers can pair up and make little dwellers, a handy way to pass on good traits and keep the vault population growing.

Romance also offers a window into Fallout Shelter’s hilarious sense of humor. Tons of cringeworthy new pickup lines were added postlaunch (“Are you a book in the Vault library? Because I am checking you out!”), which makes spying on your dwellers—hey, it’s an Overseer’s job—more irresistible than ever.

More of a pet person? You can adopt some furry friends, too. In addition to dozens of cats and dogs, Overseers can go full pirate with an annoying parrot pal. Polly want a radiation suit!