Poll Results

Total votes: 765

Concrete sets underwater: 283 (37.0%)
Time is relative: 133 (17.4%)
Ice floats on water: 99 (12.9%)
Birds evolved from dinosaurs: 59 (7.7%)
Troy was a real city: 52 (6.8%)
The night sky is dark: 51 (6.7%)
Continents move across the Earth: 45 (5.9%)
The sun is a star: 26 (3.4%)
Solar eclipses happen: 17 (2.2%)

It seems there's a lot of concrete fans out there. Time being relative is the foundation of relativity. Ice floating is a consequence of the extremely unusual property of water that it expands when it cools to freezing point - a property that means ice shelves float on the oceans, insulating the water underneath, rather than the oceans freezing solid from the bottom, which would be rather nasty.

That the sun is a star is probably my own greatest "Wow!" fact - it's a completely non-obvious thing that nobody knew until just over 100 years ago. It's taken for granted these days; kids are told the sun is a star before they even go to school (at least my kids would be, if I had any). Everyone knows it, but thinking about how much of human history went by before anyone knew it, and what led to that discovery, it just mind-blowing.

But my most underrated cool fact is that the night sky is dark. And I see it's so underrated that relatively few of you even bothered to vote for it!