Mud is a liquid or semi-liquid mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as siltstone or solid, mudrock lutites. When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds. Mud is closely related to slurry and sediment.

Mud, in the construction industry, refers to wet plaster, stucco, cement or other similar substances.

In ceramics, the making of liquid mud (called slip) is a stage in the process of refinement of the materials, since larger particles will settle out of the liquid.

Mud is similar to muck, but lacking significant quantities of humus, and often containing higher proportions of sand.

As habitat

Mud plastered home in Pakistan

Problems

Clay soil can pose problems for traffic when moisture is present. A road built upon such soil may become stable over time as the packing of the soil will make it more water-resistant. However, any attempt to grade it can be disastrous, since excess water can then enter the surface and will be worked in by traffic, transforming portions of the road into a mud bog that can trap vehicles. The typical solution in road building is to add layers of crushed stone. The stone particles will interlock and distribute the weight of a vehicle over a larger surface area. Proper drainage is also essential when low spots are encountered by the road, usually requiring the addition of culverts to pass water underneath the elevation of the street.

Buildings constructed upon clay soil must also be properly drained around their perimeter, particularly where a perimeter foundation (rather than a monolithic slab) is used. As clay will expand and soften when moisture is added, the resultant mud will squeeze out from underneath the foundation, however, in the next dry cycle it will contract, but the clay squeezed out will not return. Over a number of such cycles the foundation can sink in the moisture-cycled locations, possibly causing both wall and foundation cracks. Maintaining a constant moisture level in firm soil is important and can be effected by appropriate landscaping and landscaping maintenance. Where drainage is toward a building a French drain may be installed to route water around the building.

Saint Louis discovered problems with mud when in 2006 had a breakout of rashes after their Mighty Mud Mania Event.

As food

Haiti consumes a large variety of different non-traditional foods in an attempt to quelch hunger pains. Mud cakes are traditionally fashioned and consumed, but items such as clay and chalk can also be eaten. Due to recent increases in food prices and growing starvation in Haiti, this habit has been extended and received much media attention.