As we know, the Earth is presently on a 23.5 degree axis. My "original" thought is that the Earth was once on a 0 degree axis with the whole planet in a relatively temperate zone and then a catastrophe occurred which caused the Earth to be on its present axis.
This catastrophe, I believe (original thoughts, right) was caused by a large meteor impacting the earth in the Yucatan peninsula area where Belize is presently. (might also help towards an explanation for the sudden disappearance of the Mayans). This dramatic shift in the Earth's axis would explain why we find frozen mammoths in Siberia with unchewed food in their mouths and evidence of temperate zone flora beneath the ice caps.
This could also explain a dynamic shift in tectonic plates that could have caused a flood and possible worldwide earthquakes, volcanic eruptions with their secondary and tertiary effects causing the extinction of large animals like dinosaurs and most of the antediluvian population.
I also think the firmament was possibly a distinct solidlike covering in the inner atmosphere that held back a significant amount of water. Hence the Biblical torrents of rain coming from above and below.
Before you rip my theory to shreds, please remember this is a forum for "original" ideas. I can prove none of what I said but after years of thinking about this stuff, this is the best I can come up with.

Alex