Abstract　 　　  

  Planet Mercury was much ignored after the NASA Mariner-10 spacecraft visited briefly in the mid-1970s. Even though less than half the surface was photographed, there was a general idea that Mercury was pretty much like the Moon. The NASA MESSENGER spacecraft made three flybys of Mercury in 2008-09, then in 2011 became the first spacecraft to go into orbit around Mercury. It spent four Earth years in orbit from 2011-2015. Data from MESSENGER reveal Mercury to be a unique, fascinating world that is very different from the Moon in almost every way that can be measured, but comparisons between the two worlds are instructive and help us to appreciate the richness of planetary diversity. I will discuss highlights of MESSENGER's discoveries, including Mercury's amazing geology, water ice hiding near the north and south poles, the magnetic field, interior structure, and Mercury's strange composition.