One thing? One thing leads to more things, but if I had to start somewhere I'd say Noah's ark. Thirty million species? Even if we assume "parent" species which differentiated into the present population, tell me how these postdeluvian creatures gave rise to the current estimate in 4500 years.
If all species alive today were alive on the ark . . . Adam named them all? And the animals which have clearly evolved to live in extreme conditions (penguins, etc) somehow survived in the Garden of Eden's ambient temperatures, alongside naked humans, with all that blubber? And the animals who are carnivores with canine teeth and an inability to digest cellulose miraculously developed those features the moment Eve ate the fruit? And plants and animals grew defenses such as spines and poisons instantaneously? And humans suddenly felt discomfort, and women's pelvises became too small to birth children painlessly?
Which reminds me, as I mentioned briefly in an earlier post . . . the Tower of Babel? Despite fascinating research that shows beautiful phylogenic trees based on clear linguistic similarities traceable to about 50,000 years ago, I'm supposed to believe the Tower of Babel did it?
I forget, though. This is God we're talking about. Anything is possible because he's God. Omnipotent and such. Rules of logic don't apply.
Really, nearly every single thing about the Biblical Christianity and religion as a whole is so intellectually unpalatable to me that I'm embarrassed it didn't bother me sooner. Nearly daily I'm hit with a conceptual nugget that causes me to stop and wonder, "People [believers] are okay with that? I was okay with that?" Objections range from technical holes as mentioned above to larger issues of salvation, evil, sin, redemption, and such. It's like re-watching an old movie from my childhood now, as an adult, and seeing the creative plotwork and mature jokes that I never saw before. It's actually quite exciting.