Drilling fluid, also commonly called 'mud' or 'slurry,' is a mixture of water and additives that is used to make the horizontal directional drilling process easier. The most common additives, which usually only account for around 3% of the mixture, are bentonite and polymer — sometimes a combination of the two, depending on the ground conditions. Bentonite is a type of clay that can be ground, refined and mixed with water to make a mud-like fluid used in the drilling process, hence the term 'mud.'