Mercury is quite a resilient little planet, likely experiencing an incredibly violent impact early on in its existence that blew most of its crust to bits, leaving only a tiny shell around a massive iron core. The solar wind it is so closely exposed to blasts away its atmosphere, and yet it has survived.
It transited the sun on May 9, 2016, as it does about 13 times a century. The Mercury transit matters because its so difficult to study this planet. While other planets have plenty of active probes, getting to Mercury is incredibly tough. To reach it, a probe would have to be fired deep into the sun's gravity pull and aimed for a target that moves 30 miles per second. So for that reason, Mercury has only been visited on two occasions: a flyby by Mariner 10 in 1973, then an orbital mission by the MESSENGER probe, which orbited the planet from 2011 to 2015. Neither NASA nor any other space agency is currently planning to return.
View some incredible footage (complete with dramatic background music) presented by NASA Solar Dynamics Observatory and Goddard Space Flight Center.