The great majority of pits are simple shafts without any lateral galleries or headings. Where the bottom of the shaft is above the ground water table, adits or tunnels are driven along the illam layer but they do not extend far because of the problem of ventilation. At times adjoining pits are interconnected by tunnels. In some of the deeper shafts where the cost of excavation is high, lateral drifts are driven in all directions from the base of the shaft so that as much gravel as possible is collected through a single shaft. This is by far the commonest method of gemming and in heavily gemmed areas the ground is riddled with pits and connecting tunnels. In the early years of the century, Coomaraswamy commented "a gemmed out deniya should be seen to be appreciated; the place is turned into a rabbit-warren of pits and tunnels, and if the illam is good every pound of it has been excavated and washed".