If you want to experience BioShock Infinite at its most difficult, you need to play on 1999 Mode. This typically isn't an option on your first playthrough...unless you input a secret code. The best part? You probably already know the code.
Going back to the sensibilities of late-'90s shooters, 1999 Mode makes enemies tougher, resources scarcer, and resurrection more expensive. Normally, this option is unlocked after beating the game once, but if you're a glutton for punishment, you can unlock it right away. Just enter this button sequence at the main menu (with the "Play Game" option at the top):
Up, Up, Down, Down, Left, Right, Left, Right, (Cancel), (Confirm)
Yes ... for you Old School Gamers - this code SHOULD look familiar: It's the Konami Code!!!! (aside from the last part change)
Now, let's see how many people are CRAZY enough to try it!!!