Bonus modes

Successfully complete the indicated task to unlock the corresponding mode:

7 Days Of Hell: Successfully complete Day 55. In this mode, you must survive seven very long levels with different graphics effect.
All Weapons: Successfully complete Day 55. In this mode, you can use any weapon with infinite ammunition.
Blackout: Successfully complete Day 55. In this mode, your view is limited to a small circle around your character. The circle expands when more players are near. Also, radioactive zombies become very visible.
Chainsaw Only: Successfully complete a level using only the chainsaw. You cannot have any environmental or Zombie Bait. In this mode, you are limited to the chainsaw and Zombie Bait.
Hardcore: Survive seven days with no deaths. In this mode, you only have one life and cannot gain any more.
Turbo: Get a 100x multiplier. In this mode, everything moves faster.

Saving survivors

Stand next to survivors and throw a teddy bear in a safe direction to distract the zombies.

Achievements

Environmental Awareness (5 points): Kill a zombie using an environmental hazard in any game mode.
Gas Powered (5 points): Complete an entire arena, getting every kill with the Chainsaw.
Gun Shop (5 points): Kill at least one zombie with each weapon type in any game mode.
More the Merrier (10 points): Kill 10 or more zombies at the same time in any game mode.
Socialite (5 points): Play in an online multiplayer game.
Tour of the City (15 points): Complete each arena at least once in the same game in any game mode.
Can’t Touch This (20 points): Rack up a 100x multiplier in any game mode.
Friends in Low Places (20 points): Save 25 survivors in any game mode.
Interior Decorator (20 points): Kill 25 zombies in any game mode using environmental hazards.
A Week To Live (25 points): Complete 7 days in any game mode without dying.
Hotter Than Hell (25 points): Complete the final day of the ‘7 Days of Hell’ unlockable mode.
Anniversary of the dead (45 points): Complete the Final Day, in either Single or Multiplayer, and finish the game.