At the end of their first-ever E3 press conference, Bethesda Softworks surprised the audience with the reveal of Fallout Shelter, a mobile game for iPhone and iPad.

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Not only did they reveal the game, they released it. It's available right this second for free on the iOS App Store at AppStore.com/FalloutShelter.

It's hard to try to compete with the reveals of games like Fallout 4 and Doom, but Fallout Shelter is already quite an entertaining little game.

Set in the same universe as the games of Fallout, Fallout Shelter is a "post-nuclear strategy and simulation experience" that makes you the Overseer of an underground vault. You'll be able to choose a number for your vault and design it from the ground down, digging deep into the earth to set up a variety of rooms and places for your survivors to live.

Their happiness will be in your control, and you'll be able to equip some with weapons and armor and send them out into the Wasteland to scavenge.

It's a free-to-play game but unlike any you've played before. You don't need to spend any money if you don't want. There are timers but they can all be skipped, at the risk of causing an accident to happen.

It's certainly not expected of a big company like Bethesda to release a free game, but it is also a great primer to the world for anyone not familiar with it, and certainly will drive up anticipation.