Mound-building termites are a group of termite species that live in mounds. These termites live in Africa, Australia and South America. The mounds sometimes have a diameter of . Most of the mounds are in well-drained areas. Termite mounds usually outlive the colonies themselves. If the inner tunnels of the nest are exposed it is usually dead. Sometimes other colonies, of the same or different species, occupy a mound after the original builders' deaths. Mound structure The structure of the mounds can be very complicated. Inside the mound is an extensive system of tunnels and conduits that serves as a ventilation system for the underground nest. In order to get good ventilation, the termites will construct several shafts leading down to the cellar located beneath the nest. The mound is built above the subterranean nest. The nest itself is a spheroidal structure consisting of numerous gallery chambers. They come in a wide variety of shapes and sizes. Some, like Odontotermes termites build open chimneys or vent holes into their mounds, while others build completely enclosed mounds like Macrotermes. The Amitermes (Magnetic termites) mounds are created tall, thin, wedge-shaped, usually oriented north-south. Ventilation in mounds The extensive system of tunnels and conduits have long been considered to help control climate inside the mound. The termite mound is able to regulate temperature, humidity and respiratory gas distribution. An early proposition suggested a thermosiphon mechanism. The heat created due to the metabolism of termites imparts sufficient buoyancy to the nest air to push it up into the mound and eventually to the mound’s porous surface where heat and gases exchange with the atmosphere through the porous walls. The density of air near the surface rises due to heat exchange and is forced below the nest and eventually through the nest again. This model was proposed for mounds with capped chimneys and with no large vents constructed by the species Macrotermes natalensis. A similar model based on the Stack effect was proposed for mounds with open-chimneys. The tall chimneys are exposed to higher wind velocities compared to openings at ground level due to surface boundary condition. Therefore, a Venturi flow draws fresh air into the mound through the openings at ground level which flows through the nest and finally out of the mound through the chimney. The flow is unidirectional in the stack effect model compared to the circulatory flow in the thermosiphon model.