I just finished the airboat level. My god. I haven't had that much fun in a game in years. Simply amazing.
As a side note, this whole airboat (and I'm sure the buggy section) really shows how dynamic Source is. You could build an entire game around JUST the airboat very easily. Awesome.
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http://www.oceas.com/SpreadFireFox
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He cut the possum's face off then cut around the eye socket. In the center of the belt buckle, where the possum's eye would be, he has placed a small piece of wood from his old '52 Ford's home made railroad tie bumper. Damn, he misses that truck.