In 1900, while drilling an oil well in Spindletop, Texas, workers ran a herd of cattle through a pit filled with water. The mud that resulted was pumped into the borehole. While drilling fluids still are called mud, engineers no longer rely only on water and clay. Instead, they carefully design compounds and mixtures to meet specific needs under various drilling conditions.
Long ago, people generally were drilling for water, not for oil. In fact, they were annoyed when they found oil by accident because it contaminated the water. It wasn't until the industrial revolution of the 19th century that drilling for oil became widespread as industrialization greatly increased the need for petroleum products.