The San Bernardino Ossuary in Milan - these are actual bones, displayed this way when the cemetery filled up, so they were exhumed to make room for other people. They chose to display them this way, beginning with the 17th century, as a way to remind people of "memento mori" - Latin for "remember that you must die".
P.S. the fresco, painted in the last 1600s, displays the soul's ascension to heaven


















