The single pit latrine is one of the most widely, cheapest and easiest used sanitation technologies. It is made out of a latrine superstructure, a slab and a pit. Excreta, along with anal cleansing materials (water/solids) are deposited in the pit. To avoid people falling in the pit and increase convenience and reduce odour, a slab with a hole is used to cover the pit. Lining the pit prevents it from collapsing and provides support to the superstructure but the underground of the latrine should be water pervious. When the latrine is filled up, it needs to be emptied or closed and relocated. By constructing twin pits (the double pit latrine), it should be possible to dig out a filled pit, after it has stood for a year, without any objectionable smell, whilst the other pit is in use. Pit latrines are part of the improved sanitation options. However, odour and fly nuisance are very common and due to soil infiltration there is danger of groundwater contamination, especially in densely populated areas.