Many of us have an instinctual connotation with the term "unmarked grave" to that of a down-on-their-luck, penniless, pauper buried in an unmarked hole in the ground to be forgotten. And while that is true in some cases, the term encapsulates much more and includes names we would never have imagined.
Mourning a loved one is supremely personal. When a loved one was also admired by millions of people around the world, the presence of their fans can create unnecessary tension and added grief to the mourning family. Which is why even some of the largest personalities in life may have small, unremarkable memorials in death.