ALH84001
ALH84001 is the designation of a meteorite fragment, generally regarded as originating on Mars. It was discovered in December 1984 in Antarctica by a meteorite-search expedition of the ANSMET program, as one of 7000 others. On discovery, its weight was 1.93 kg. According to NASA, it was formed on Mars from molten lava about 4 billion years ago, then came to earth as another meteorite impacted on Mars, throwing ALH84001 off Mars' surface as debris, about 15 million years ago. Roughly 13,000 years ago, ALH84001 impacted on Earth.
ALH84001 became known because of certain microscopic structures that were for some time considered to be the remains of bacteria-like lifeforms. The structures found on ALH84001 are 20-100 nanometres in diameters, similar in size to the hypothetical nanobacteria, but smaller than any non-hypothetical earthly lifeform. If the structures are really fossilized lifeforms, they would be the first proof of extraterrestrial life and, potentially, the panspermia theory. Today, the actual origin of the tube-like structures is still a subject of dispute.