Board Thread:Questions and Answers/@comment-27042328-20151004164350/@comment-995426-20151006124258

MiwandSplinterfan1: The best theory I've heard so far relates to an easily overlooked scene in Vengeance is Mine. Baxter accidentally spilled a mysterious chemical in his mutagen formula for the snake mutation. It is suspected by a lot of fans that Baxter was working on his own retro-mutagen, and whatever he'd been working on (whether complete or incomplete), was what fell in.