Chunks of rock from the Martian surface occasionally collide with our planet. By 1997 twelve meteorites were positively identified as being from Mars. They are called SNC meteorites (named after the first 3 meteorites found: Shergotty, Nakhla and Chassingy). Dr. Colin Pillinger of the U.K. Planetary Sciences Research Institute says, "100 tons of Martian material arrive on Earth each year."
In 1984 Martian meteorite ALH84001 was discovered in Antarctica. In August 1996 NASA scientists identified tiny tubular structures in ALH84001 as being, "possible microscopic fossils of bacteria-like organisms that may have lived on Mars more than 3.6 billion years ago." In October 1996 British scientists announced that a 2nd meteorite from Mars, EETA 79001, contained the chemical signatures of life, in this case, "organisms that could have existed on Mars as recently as 600,000 years ago."