A new update available now for Fallout Shelter on Android and iOS has added a super-challenging Survival Mode to the game, along with an extensive list of new features.
Starting a new Vault from the main menu, you can start a new game with Survival Mode turned on, described as Vault-Tec's "most rigorous and hazardous social experiment yet." Not only will you "face deadlier threats," but you'll need to generate more resources to sustain your Vault, and you won't be able to revive dead Dwellers.
So you better make sure you have a healthy stock of stimpaks and that you don't leave your explorers out in the wasteland too long. And like Hardcore Mode in Fallout: New Vegas, you won't be able to turn back and deselect Survival Mode once it's been activated. Makes me wish I hadn't bought all those lunchboxes on my regular Vault.
Apart from this, your saves can now be catalogued on the cloud for easy storage between devices. Dwellers can now scavenge equipment from dead Raiders so that Raider Attacks aren't a boring exercise anymore by the endgame. You can also find a mysterious Dweller in the Vault and you will earn extra bottle caps for positive identification.