Crawler Zombies (also known as Gas Zombies and in Dead Ops Arcade, Creepy Crawlers) are failed experiments that were created by Group 935 with Element 115. When killed by something other than the Ray Gun or Knife they will release Nova 6 Gas, which can disorient a player. Crawlers only appear in the maps Kino der Toten, and "Five". In Kino der Toten, you must turn on the power before the crawlers can come.
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Crawler zombies also appear in Magma-Man's map, Usopire, and also in Dead City. However, they are not the same in these maps. Instead of blowing up into Nova 6 gas, they move twice as fast as a normal zombie, but have half the amount of health. They do not emit Nova 6 gas. They're main purpose is how they can climb on wall, which allows them to climb to the barriers on the second floor of the building in Usopire or climb up the skyscrapers and other building to get in in Dead City.