Meteorite ALH84001 is a Martian meteorite that fell to Earth in Antarctica 13,000 years ago. Researchers caused a storm of controversy in 1996 when they announced the discovery of structures in the meteorite that looked like fossil bacteria - the strongest evidence yet for the existence of life on Mars. The orange stains in this macrophotograph, 100-200 micrometers across, are carbonate minerals that precipitated out of water some 3.6 billion years ago, when Mars was a geologically active planet with a thick atmosphere and surface water. It is these carbonates that contain the bacteria-like structure.