An illustration depicts the possible extent of an ancient lake inside Gale Crater, where the Mars rover Curiosity landed on the Red Planet in August 2012. The $2.5 billion NASA mission set out to explore Gale Crater, which was thought to have once hosted flowing water. Curiosity found evidence of clay formations, or "mudstone," in the crater's Yellowknife Bay, scientists said in 2013. This clay may have held the key ingredients for life billions of years ago. It means a lake must have existed in the area.