ValVe has sent us gamers another challenge, play the Terminal map for Dead Air in versus mode 60,000 times. Unfortunately a lone gamer cannot complete this as 60,000 people need to complete a full round in Dead Air and get the achievement Connecting Flights. So come on people, we know you love killing zombies as much as I do, LEND A HAND! Log in, play a few rounds and let’s get this thing unlocked!
Source Link: Left 4 Dead Blog