Several years ago, NASA sent its Messenger probe to Mercury, and for months now it's been in orbit, gathering data, and beaming the information back to Earth.  New information has so far revealed that there are "cold pockets"  that may contain water on the otherwise extremely hot Mercury, evidence of a crust-shredding collision, and intriguing data about Mercury's unusual orbit around the Sun, which could eventually affect the orbit of Earth and other planets.

Image credit: NASA