But besides small size and swift speed, little was known about this mysterious planet until NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft became the first man-made instrument to orbit Mercury in 2011.
On Thursday April 30, NASA will crash land the spacecraft into the planet, leaving a 50-foot-wide crater. But before meeting its violent end, the spacecraft and its seven scientific instruments have uncovered many of the alluring planet's secrets -some, however, continue to defy understanding.
Here's a collection of some of the most incredible photos the spacecraft took that helped scientists better understand this small world. Some of these images have false coloring to highlight different minerals and features on the surface.