Besides the coffins themselves, some other curious features are found in the burial-places. The dead are commonly buried, not underneath the natural surface of the ground, but in extensive artificial mounds, each mound containing a vast number of coffins. The coffins are arranged side by side, often in several layers; and occasionally strips of masonry, crossing each other at right angles, separate the sets of coffins from their neighbors. The surface of the mounds is sometimes paved with brick; and a similar pavement often separates the layers of coffins one from another. But the most remarkable feature in the tomb-mounds is their system of drainage. Long shafts of baked clay extend from the surface of the mound to its base, composed of a succession of rings two feet in diameter, and about a foot and a half in breadth, joined together by thin layers of bitumen. To give the rings additional strength, the sides have a slight concave curve and, still further to resist external pressure, the shafts are filled from bottom to top with a loose mass of broken pottery. At the top the shaft contracts rapidly by means of a ring of a peculiar shape, and above this ring are a series of perforated bricks leading up to the top of the mound, the surface of which is so arranged as to conduct the rain-water into these orifices. For the still more effectual drainage of the mound, the top-piece of the shaft immediately below the perforated bricks, and also the first rings, are full of small holes to admit any stray moisture; and besides this, for the space of a foot every way, the shafts are surrounded with broken pottery, so that the real diameter of each drain is as much as four feet. By these arrangements the piles have been kept perfectly dry; and the consequence is the preservation, to the present day, not only of the utensils and ornaments placed in the tombs, but of the very skeletons themselves, which are seen perfect on opening a tomb, though they generally crumble to dust at the first touch.