A colossal valley near Mars’s equator is revealing dramatic clues about the Red Planet’s watery and volcanic past. Stretching roughly 1,300 kilometers, Shalbatana Vallis was carved billions of years ago when enormous floods of groundwater burst onto the surface, gouging deep winding channels across the landscape. Today, the region is a striking mix of ancient flood scars, collapsed “chaotic terrain,” lava-smoothed plains, volcanic ash, and battered impact craters — all hinting at a Mars that may once have been far warmer and wetter than it is now.