What cultural practice involved the Greeks calling marked graves a specific term related to swelling ground?

The term related to swelling ground that the Greeks used to refer to marked graves is "tumulus." This term specifically describes a burial mound or earthwork that is built over a grave or graves. The word itself derives from the Latin root meaning "to swell" or "to heap up," which is quite fitting as tumuli are often large and prominent features in the landscape, visually rising above the surrounding ground. This practice illustrates the importance of the burial site in ancient Greek culture, serving both as a physical marker of the deceased and a symbolic connection to the earth.

While tholos refers to a circular tomb structure often covered by a dome, kylix is a type of cup or drinking vessel, and stelae are upright stone slabs used for marking graves or commemorating the dead, they do not directly relate to the swelling ground aspect that defines a tumulus. Therefore, "tumulus" is the most accurate choice, as it captures the essence of the burial practice characterized by an elevated grave space.