Volcanic Explosion on Mars Created Weird Formation

NASA's Mars rover named Spirit collected data around a table-top-like formation. The analysis of this data shows that the material was once volcanically ejected, then buried beneath finer materials. This finding is huge in that it is the first strong evidence to show explosive activity based on materials found on Mars. Especially of interest was a "bomb-sag," which is a small depression that a falling rock would have created. Additionally, rocks in the deposit are found to be basaltic, which is volcanic in origin. One way to get basalt is to have lava come into contact with water, so this finding could be strong evidence for there once being water on Mars.

http://news.nationalgeographic.com/news/2007/05/070503-mars-volcano.html