A big part of my awakening was learning in Native History classes in university of the existence of mammoth kill sites in North America with spear points still embedded in the bone. These sites are reliably dated to almost 13,000 years ago. I'm Native American and I couldn't wrap my head around the idea that the flood supposedly wiped out the mammoths (as the watchtower says) yet there were clearly people hunting them. Would Jehovah(tm) be such an unloving bastard as to wipe out an entire people who never even knew what was coming? Unless Noah magically "preached" in North America, too.
Of course, subsequent classes in geology, atmospheric science and anthropology wiped out any remaining shred of belief in a global flood.