Horseshoe crabs are not much to look at. But beneath their unassuming catcher’s-mitt shell, they harbor a half-billion-year-old secret: a superpower that helped them outlive the dinosaurs and survive all the Earth’s mass extinctions. And what is that secret superpower? Their blood. Their baby blue blood. And it’s so miraculous that for decades, it hasn’t just been saving their butts, it’s been saving ours too.