NASA's Mars Reconnaissance Orbiter (MRO) has taken detailed pictures of what scientists are saying is evidence that large lakes of liquid water sat on the planet's surface relatively recently--which is to say, about 3 billion years ago. MRO imaged several deep depressions that scientists previously attributed to the sublimation of underground ice 4 billion years ago. However, the new images show that the depressions are connected by long channels, and researchers say