Note to Hamlet, 1.4.48-50: "why the sepulchre . . . Hath oped his ponderous and marble jaws"
A sepulchre, according to the Oxford English Dictionary, is a "tomb or burial-place, a building, vault, or excavation, made for the interment of a human body," but the phrase "ponderous and marble jaws" makes me think that Shakespeare has in mind something like what you can see in the photograph below.
Sepulchre of János Hunyadi (d. 1456)
Source: Historical Text Archive: Images of Transylvania, Part II