Bethesda's Fallout Shelter, the mobile game tie-in to the upcoming Fallout 4 where players manage an underground vault full of nuclear apocalypse survivors, made more than $5.1 million in its first two weeks on the iOS App Store according to sales tracking organization SuperData. Perhaps even more impressively, that moolah is all from optional in-app purchases, as the game is free to play.

For some context, Fallout 3, the last Bethesda-developed go-round in Fallout's unique '50s-style wasteland, sold 610,000 units in its first month according to the NPD Group. Cut that in half (which is the best way we can estimate its two-week sales, since such data is otherwise heavily-guarded by publishers) and we get $18.3 million made, assuming each copy was sold for $60. Fallout Shelter's two-week launch would be just shy of one-third of that amount.

So if you ever find yourself asking, "why is (your favorite developer) focusing on mobile games," making $5.1 million in less than two weeks might be part of it.

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