Another interesting feature of the Great Pyramid are the so-called 'relieving chambers'. This is an ingenious system raised over the king's burial chamber, constructed to relieve vertical and horizontal loads. The relatively thin wall between the burial chamber and the grand gallery likely could not absorb all of the horizontal pressure from the granite roof of the chamber. Thus, the granite roof had to be raised above the level of grand gallery. However, this created another problem, since now the walls of the burial chamber would be more than 8 meters high, and thus very unstable. Thus, huge granite slabs were placed horizontally as stabilisers between the walls, one above another. The empty spaces between granite slabs today are called 'relieving' chambers, though they were never meant to be accessed; and were not until the 19th century, when they were opened by dynamite. This, turned out to be an incredibly valuable discovery. Since the 'relieving chambers' were never meant to be accessed, the stone blocks on their walls were not prepared and laid as carefully as those in the accessible interior of the pyramid. Thus, graffiti of the pyramid workers were found inscribed on some of the blocks, mentioning the names of the worker crews which built the pyramid (or at least placed these particular stones), and the year of rule and the name of the pharaoh who had the pyramid built: Khufu. The Great Pyramid is otherwise completely empty of any inscriptions or hieroglyphs whatsoever. This is the only evidence which proves that pharaoh Hor Medjedu Khnum Khufu had the Great Pyramid built, and that this is, in fact, the legendary king which Herodotus calls Kheops.