In the late 70's meteoriticists began to speculate that some oddball meteorites might be from Mars. These meteorites looked more like terrestrial rocks than meteorites, but they were seen to fall- Chassigny in 1815, Shergotty in 1865, and Nakhla in 1911. The big question, how these SNC meteorites got here, seemed to be solved by the "spall mechanism", a combination of high velocity and low shock pressure. Although the "spall mechanism" is shown to be spurious, trapped Martian atmosphere and isotope ratios provide convincing evidence that some 67 (as of 8 June) distinct meteorites are from Mars.
George Wetherill's mechanism, acceleration by the expansion of shock-compressed water is shown to be a viable means of getting meteorites from Mars to the Earth. Since the "spall mechanism" is still generally accepted by the Planetary Science community, it will be discussed in detail and shown to an artifact of computer code calculations. There is also evidence from the shock-induced high pressure phases in some martian meteorites that they were too deep in Mars to have been launched by the "spall mechanism".