After being batted around the solar system like some cosmic softball, a rock from Mars ended up on Earth thousands of years ago. Now, 17 years after its discovery, it has become science’s most studied stone. Researchers say the rock, known as the Allen Hills meteorite, provides compelling evidence that there was life on Mars, at least in the distant past. Few questions loom larger in space science today. If Mars once had life, than we humans would be faced with the increased likelihood that life has sprung up elsewhere, and we that are not alone. But after five years of intense scrutiny, the scientific community is no closer to agreement on whether the rock tells us anything.