A rock containing many smaller round stones could indicate that there was once a large amount of liquid water on the Red ...

A weathered sign in the Minnesota River Valley proudly proclaims: “World’s Oldest Rock.” Erected in 1975, it marks a 3.8-billion-year-old gneiss — or so scientists thought.

Imagine a gentle landscape of rolling plains and peaceful forests, stretching as far as the eye can see. Now, picture that ...