For about six months, NASA's Curiosity Mars rover has been exploring a region full of geologic formations called boxwork, low ridges standing roughly 3 to 6 feet (1 to 2 meters) tall with sandy hollows in between. Crisscrossing the surface for miles, the formations suggest ancient groundwater flowed on this part of the red planet later than scientists expected. This possibility raises new questions about how long microbial life could have survived on Mars billions of years ago, before rivers and...